Who would have a standard design that didn't allow for wallboard on the ceiling? That doesn't sound like a "standard" that would get called for much. I realize that trusses can take a load from above that is different from a hanging load. But I'm just thinking that any design would be able to handle the minimal load of wallboard. Of course this is on 48" centers so that increases the load a bit. You could probably add some vertical 2x4's to transfer the hanging load to the top at strategic locations and make it work even if they can't handle it.
Still, all in all sounds like a poorly thought out project based on all the subsequent things the op has asked about doing to it.
I agree on the project not being thought out..It sounds as if he had one of those El-Cheapo package garages thrown up with very minimal framing..Stupid move if he planned on finishing it...Imagine what that roof is gonna look like in a few years with 4 foot on center trusses.And the soffits..And the walls....LOL...