Include `meta` member in resource identifiers in `relationships` object - active-model-serializers

Question

From my understanding of the JSON:API spec (specifically https://jsonapi.org/format/#document-resource-object-linkage) I should be able to include meta members for each member of a relationship.
I have been able to add a hash of meta data to the relationships object itself, but not one to each of the individual relationships within.
class PlanSerializer < ApplicationSerializer
attributes :id, :name
has_many :features do
meta value: "x"
end
end
I know I can use a block syntax for has_many, and think that's the way to achieve this. But I haven't got it working. Calling the meta method within the block adds the meta block to the features relationship object, and I need to add one to each entry in that array.
My questions:
Have I understood the spec correctly? Should I be able to add a meta object to each relationship?
How would I go about doing this with the active model serializers?
Background:
My goal is to represent a many-many from Plans to Features where each plan might have some extra information for it's own relationship to a given Feature (and that information is different for every Plan, so it doesn't belong on the Feature object)
If your answer is that I shouldn't be doing this, that's fine, but please present an alternative which you think is preferred.
// My desired output
{
"data": [
{
"id": "small",
"type": "plans",
"attributes": {
/* Some attributes */
},
"relationships": {
"features": {
"data": [
{
"id": "num-users",
"type": "features",
"meta": {
"value": 1
}
},
{
"id": "num-projects",
"type": "features",
"meta": {
"value": 5
}
}
]
}
}
},
{
"id": "large",
"type": "plans",
"attributes": {
/* Some attributes */
},
"relationships": {
"features": {
"data": [
{
"id": "num-users",
"type": "features",
"meta": {
"value": 5
}
},
{
"id": "num-projects",
"type": "features",
"meta": {
"value": 50
}
},
{
"id": "unlimited-downloads",
"type": "features"
}
]
}
}
}
],
"included": [
{
"id": "num-users",
"type": "features",
"attributes": {
"description": "Number of users"
}
},
{
"id": "num-projects",
"type": "features",
"attributes": {
"description": "Number of projects"
}
},
{
"id": "unlimited-downloads",
"type": "features",
"attributes": {
"description": "Unlimited downloads"
}
}
]
}