In continuing to search, I think I've answered my own question, so I thought I'd post it here, to help future searchers! I'm unable to share links, but I found what appears to be UNA developer documentation (at ci.una.io/docs), and there is a "service call" in that documentation under "Base Profile"/"Browse" - "browse_by_acl":

Browse profiles by membership(ACL) level

So I believe that the functionality I described above will be possible for us to add to our site if/when we're able to fundraise the cost to hire a developer.