Query User Picker fields by role

The comment functions have a really useful inRole predicate that lets you search on wether the commenter was in a particular project role. It would be nice to extend this functionality out to user picker fields, so that you could say things like assignee in role or "My User Custom Field" in role. \

Notably, the inGroup predicate is already supported by JIRA's native JQL. It's just role searching that's really needful.