I think alliances should be able to pay for their own alliance gear, yes. But I don't think it should be custom, because that would be a huge workload on Saroman to implement.
Instead, I think there should be a generator similar to this: http://www.furiousn00b.com/halo4/emblems/ with many, many wurm-related options, unlimited color choices, and different textures, designs, etc. to pick from.
Gives alliances unique gear without overloading the art team and the database with custom skins for every alliance out there, and it would be easier to add to the game in the long run, I'd imagine.