Triggering class-level bean validation through AJAX in JSF2/PrimeFaces - jsf

Question

Problem description
At the company I work for we're implementing a web application using JSF2 and PrimeFaces. The web app is one of the front-ends hitting a bunch of business methods and a domain model created as a set of entity classes and persisted using JPA. We also have a couple of webservice methods operating on that same domain model. Because of this we want to implement as much of our validation logic as possible using bean validation on the entity level. So far everything's working out quite nice but we recently bumped into a problem I don't really know how to deal with. To improve user experience we trigger most of the bean-validation logic using AJAX (using p:ajax update="errorMessageForFieldWhatever"), for example when the user tabs out of a text field, changes a value in a dropdown, etc so that they have immediate feedback about the error. This all works fine when dealing with field/property-level constraints but I don't see how to make it work properly with class-level bean validators. Let me illustrate with an example.
Assume the following entity object, CDI bean and facelets view, and a custom bean validator which requires that MaxValue is greater than MinValue.
#Entity
#CustomClassLevelBeanValidator
public class Model {
#Min(10) int minValue; int maxValue; // getters/setters ommitted }
#Named
#SessionScoped
public class ModelBean {
#Valid private Model model; // getter and initialization ommitted }
<f:form>
min value:
<p:inputText id="minValue" value="#{modelBean.model.minValue}">
<p:ajax process="#this" update="minValueErrorMessage"/>
</p:inputText>
<p:message for="minValue" id="minValueErrorMessage"/>
max value:
<p:inputText id="maxValue" value="#{modelBean.model.maxValue}"/>
<p:message for="maxValue"/>
</f:form>
What we want to achieve is the following:
When the user tabs out of minValue, the error message for that field gets updated. This already works because of standard JSF/single-field-bean-validation integration.
When the user tabs out of EITHER minValue or maxValue, the error message for maxValue should be updated. Note that this really consists of 4 separate cases: the constraint can become valid as well as invalid through changes in minValue and the same goes for maxValue. I'm not clear how to make this work without resolving to JSF-level validation.
Current state of affairs
Direct updating of single-field error messages on ajax events already works (out of the box).
By making use of MyFaces' ExtVal component we also managed to trigger class-level validations on form submit, although all constraint violations end up in the "global errors" section (p:messages globalOnly, which makes sense since during class-level bean validation you do not specify which property failed validation).
We already implemented a solution which is functionally equivalent to what I lined out above (from a user's perspective) but I hate it. It involves a lot of process=this/update=that on the facelets side and sometimes the use of JSF-level validation thereby violating DRY since we'll have to repeat those constraints in the domain model again to make sure webservice calls are properly validated, too.
If it turns out that what we want to achieve is not possible/feasible we'll have to settle for triggering field-level constraints through AJAX and process all the cross-field stuff on form submit. It's not that bad actually but I'm hoping we can do better. Coming from a .NET background I remember this kind of stuff being reasonably easy to implement using WPF and IDataErrorInfo.
Solution requirements
An ideal solution would satisfy all of the following requirements:
Be fully implemented using Bean Validation alone, no FacesMessages etc
Allows direct feedback to the end user after editing a form field, on validation errors on that specific field and all other fields whose constraints are affected by it
Shows validation errors "where they belong", e.g. in the above example the rule "max > min" is, at least from a user's perspective, tied to the "maxValue" field. The fact that such a constraint is not strictly an error on maxValue but rather a relation between both fields doesn't really matter, I should be able to pick one of the two as the "victim" for validation and present the end user with the message "sorry, that specific field is wrong".
I understand that this is not possible in the general case having constraints over N fields some of which may not even be in the current view, but I think stuff like min > max, endDate > startDate etc could be covered.
Where to go from here?
As far as I'm aware there's nothing in JSF, BeanValidation or PrimeFaces that let's me achieve this. I'm not so sure about ExtVal, it seems designed to be very extensible but even then I wouldn't know where to start. If there's anything in any of these libraries which I completely overlooked and that let's me solve this problem, please let me know!
If not, what would it take to build a custom solution to this problem? I've thought about manually implementing this, something along the lines of a custom phaselistener which triggers all bean validators for all submitted fields in the current views and turns them into FacesMessages. However I suspect this will not be an easy task:
Standard class-level ConstraintViolations don't carry a leafBean/property-path, without that, validation error's probably can't be matched to jsf's client-ids
JSF does not apply model values if any of them fails validation. In cross-field validation scenarios it is possible that applying a single value violates a constraint, while applying all of them would make the object valid again (how does ExtVal do this? does it not follow JSF's rules?)
Do we validate during PROCESS_VALIDATIONS? If so, should we enable DISABLE_DEFAULT_BEAN_VALIDATOR context param to allow otherwise invalid model values to populate the entity?
It seems part of the problem is that JSR303 sees constraint validation as a state (an object is either valid or not) while JSF sees it as an action (no, you can't submit this form, it's invalid). Will JSF2.2 make life easier in this regard? I wouldn't mind a user submitting invalid values, we'll just make sure not to store them in the DB ourselves. At least this solves the problem of having to reset UIInput components.
The longer I think about this the more I suspect it's just not going to work the way I want it to. Still I feel kind of stupid having to tell our users that "no sorry, end date must be after start date is such a complicated business rule that we cannot give feedback directly, you'll only bump into that error when you submit the entire form." So if anyone comes up with a solution which fullfills all requirements I'd be very grateful to hear about it.