Custom "assigned" type fields...?

Don’t know if I’ve missed something obvious in the docs, but I want to make custom issue tracking for different types of issue, and restrict the choices of assigned user to only people who would be valid for the class of issue in question.

If I make an “issue” type in Module Builder, it gets an “assigned to” entry that isn’t visible in the fields.

Is there some way for me to get in and edit the assigned so that it only lists users who either have a particular role or have a particular value in a particular column (e.g. has_PVG_check IS TRUE)?