Somebody mentioned asking the fire marshal, which isn't a bad idea, but I would point out that sometimes they can be very rigid in their rules. Once you tell them you want to run a generator inside your building you may find them putting all kinds of requirement on you such as powered ventilation fans and only going up and out the roof and things of that nature. What you are talking about doing is not that complicated and what ever you use for piping should be fire proof, but it isn't really going to get that hot. I'd go with something like what some have already suggested and then if you have a building inspection coming up just remove the piping before the inspection.
