I would absolutely buy additional costume slots for my BOFFs. Agreed, it should be account-wide, the same way additional costume slots for captains work.
Personally, I would want at least 3 slots for a duty uniform, a field uniform, and a dress uniform.
A BOFF on one of my toons has the bug too. He's got a second costume slot, but there's now way for him to "wear" it. Based on the existence of this glitch, I'm guessing that at least some of the programming to support this has already been done.