I’m trying to do exactly the same.
Initially I override the user-permissions plugin, by creating the endpoint and a new handler for this endpoint. i.e
In my case I created a field Personal Trainer, for the user (a user can have a PT). then I created the route and the handler but for my surprise it is not called at all… for what I can see the UI / Strapi call an internal API, content-manager
POST /content-manager/relations/plugins::users-permissions.user/personal_trainer?_limit=20 (535 ms) 200
Is there any way to do this? I believe filtering on relationship it is a pretty much extended use case, i.e show food that is not expired… etc etc