At the moment, using either team prefixes/suffixes or the BELOW_NAME display slot would be your best options. However, each has some downsides:
- Prefixes and suffixes, as their names imply, can only alter the beginning and end of a player’s name. Depending on how you want to alter the player’s name, this could pose a problem
- The
BELOW_NAMEdisplay slots places the player’s score for the objective after the text. This is done client-side, so you’ll always end up with a number after your guild tag.
In the future, Sponge willl most likely support modifying the full above-head display name for players, just as you can for humans. Until the, you’re best off going with your original plan.