I think this forum is probably the best to come to for this type of advice.
I had a crazy idea yesterday. Let me first start out by saying that I live with 2 guys, we all know what we're doing mechanically, one is even a mechanical engineer, so he's "qualified". That's not the part I'm worried about.
I have this hideous couch. We don't want it, but neither does anyone else. The cushions are nigh-on ruined. I had a thought a few weeks ago to turn it into a couch-cycle, and go cruising through campus on it. Yesterday, I remembered my dream of roadtripping with my friends, and my other dream of bike touring. My thoughts turned to the couch, but that was simply not grandiose enough. So after a late night, I have some basic plans for an RVcycle, with 3-5 stations for pedalling, a bed in the back, etc.
So, I don't think it will go very fast. It will need to be able to go up hill, after all. And it will be big, bigger than many cars. I figure this forum knows more than any of the others about slow-moving vehicle laws. I think that as long as I have proper lighting, the orange flags, and a dot-yellow "slow-moving vehicle" sign on the back, I should be fine, right? I could think up a way to rig brake lights, but I don't think they're necessary from a safety standpoint, and they would be a hassle.