I remember playing a map many years ago, called "Snipers: Requiem". It's a typical snipers game, but with a few peculiarities. One of them that stood out to me was the use of a custom portrait in the game. I happened to find a gameplay video of that map that shows it:

First of all, is that a fully custom portrait or just a portrait for one of the units that never made it past the beta stage?
Secondly, this is a multiplayer map that was made more than 8 years ago. Were EUDs with such capabilities a thing back then?
If not, then how could custom portraits be used in multiplayer without EUDs?

It's a built-in unused portrait.

Can't answer the EUD questions.

EUDs were definitely around 8 years ago, but this was probably done with extended portrait IDs and custom unit portraits with EUD conditions.
I have no idea if extended IDs still works in SC:R, but modifying portraits.dat with EUDs should work and is probably a better solution. http://farty1billion.dyndns.org/euddb/?pg=entry&id=635 http://farty1billion.dyndns.org/euddb/?pg=entry&id=636