Acl rules don’t support this operation. You can try adding another formula column containing all users that are in that region, and then in ACL rules use the in operator. For example:
# Users column:
list(u.Email for r in $Region for u in Users.lookupRecords(Region=CONTAINS(r)))
# Acl formula
user.custom.Email in rec.Users
Yes, this document describes what is allowed.
I don’t think there are any limitations to adding this feature. All ACL formulas are handled and compiled in this file, any improvements are welcomed