I have a suggestion to make this work for DarkRP and TTT.
As DarkRP uses player teams for, well, jobs (as far as I am aware) and TTT uses teams for, well, teams... whats stopping you from just going straight to where sandbox gets its teams from and... say... set the tag based on a players rank in ulx? Maybe add something like how DarkRP uses an entire LUA file for adding entries to a table so server owners can add a label for a particular rank, and just show nothing if the player being targeted has no rank/doesn't have a rank that matches up with the table?
Surely it can be that easy?