Update JSF messages after initial rendering via p:menuitem - jsf

Question

Does anyone know if it's possible to update a JSF message or messages element AFTER its initial rendering via PrimeFaces' p:menuitem?
For example, I have a PrimeFaces p:menuitem I am using to open a p:dialog, and that p:dialog has a p:messages element whose message I would like to update and show the moment that the p:dialog is opened - I can't know whether there is a message I want to show or what that message should be until after the p:menuitem's action method is complete.
Unfortunately, setting the update attribute of the p:menuitem to both the id of the p:dialog and the id of the p:messages element itself does not cause the p:messages element to show its message, even though I think it should since the action method of the p:menuitem looks like it is properly adding a message to the p:messages element.
The menuitem opening the dialog:
<p:menuitem value="Show Dialog with Message" action="#{myView.prepareDialogWithMessage()}" oncomplete="PF('dialogWidget').show()" update="dialogWithMessage message" />
The dialog with the message:
<p:dialog id="dialogWithMessage" widgetVar="dialogWidget" resizable="false" dynamic="true" closable="false" showEffect="fade" hideEffect="fade">
<p:messages id="message" for="message" showDetail="true" escape="false" autoUpdate="true"/>
<div class="button-panel">
<p:commandButton value="Yes" styleClass="ui-confirmdialog-yes" action="#{myView.submitAction()}" oncomplete="PF('dialogWidget').hide();"/>
<p:commandButton value="No" styleClass="ui-confirmdialog-no" onclick="PF('dialogWidget').hide();"/>
</div>
</p:dialog>
The Java code which adds the message:
public void prepareDialogWithMessage() {
FacesMessage message = new FacesMessage(FacesMessage.SEVERITY_WARN, "Summary of message we want to show when the dialog opens", "Details of the message we want to show when the dialog opens");
FacesContext.getCurrentInstance().addMessage("message", message);
}

Answer 1

Short answer: It behaves exactly as expected and implemented.
Longer answer:
Having a closer look after your edited question, I noticed the dynamic="true" on the dialog. If you read the PrimeFaces docs it states (as you already found out):
dynamic: Enables lazy loading of the content with ajax.
So when you show the dialog via javascript, its content is updated... Since this content also includes the p:messages, that is also updated. In your case updated again with the messages that are generated in the ajax call of the update of the dialog... Most likely none... So the messages you added to it in the prepareDialogWithMessage() call of the menuItem are lost.
Since you already update the dialog in the call to the menuItem, the dynamic='true' is totally superfluous and it throws a spanner in the works.
<off-topic>A suggestion is to always start creating an [mcve]. Removing more and more until the behaviour of certain parts is as expected. Removing the dynamic='true' in one of the steps would have resulted in this and you'd have known a lot more. At the same time, also try debugging more. Investigating by looking at the requests and responses in the browser developer tool. You would have seen the messages getting added to the dialog and then the dialog(contents) being totally overridden. All these things are easy too (sorry, easy instead of 'not to difficult), can/will tell you a lot and helps getting to the cause of things quicker. Either directly, or by being able to ask way more specific question that is often easier to answer</off-topic>