From an artwork standpoint, it's not as bad as it sounds. A number of games, notably MechWarrior 4, use custom image tags on their models to denote alliance (in addition to the myriad paint options).
The images are downloaded on an as-needed basis, before each match I think. What that would mean for PS would probably be "per load area", so newcomers logging on in Hydlaa would probably need to hop out to the Library or Forest maps and back to reload. Alternately, and admittedly the better option, would be to DL the logo images based on field-of-view (or at the least, camera range). The trick becomes how to make it work efficiently - you don't want to bog down the client with heavy FOV code for a gimmick.
Art-wise, we already store logs and screenshots locally. Makes sense that you can stash your guild-tag image, and download others, into the same area.
The DOWN SIDE of this is, it then becomes all-too-easy for some mischief-maker to go tinkering around with those images and post something obscene or racist, which would then get loaded onto the machine of anyone who saw/anyone who loaded the area they're in. Most of us are mature enough not to abuse such a feature, but there's always the exception... that said, I would definitely support the "template" idea mentioned in the OP, as it would be much more difficult to create offensive content.
