Submit to me or to CONLANG a design for T-shirts. This would probably also be used for the cover of the conference packet.
I am not sure if I can swing it at this point, but if I can: the winning designer gets a free one.
Feel free to incoporate the Conlang Flag, or to make up a name for the conference as you like. The official sponsor group is the Language Creation Society, which is only allowed to be referred to as a "berkeley campus group" (if at all referencing Berkeley). It has no logo at present; feel free to create one, or omit it that affiliation entirely, as you like. The one thing you can't include is any UC / Berkeley / Cal logo or trademark whatsoever - they're really picky about the use thereof.
Future conferences may or may not be held at UC Berkeley - there is some talk of a potential for its continuation elsewhere.