As to the original question, you can use whatever code you feel comfortable using, so long as the AHJ approves. I think it would be a mistake to stick to the old code under any circumstance (unless required by the aforementioned Podunk, TX building official), since doing so assumes that the...
IBC 2000 Section 1622 covers "Nonbuilding Structures Seismic Design Requirements", which would seem to apply. (2003 should be similar, but 2000 is open on the desk.)
The AHJ would have the final word on what and how it is applied. Note that S.1622.1 of the Code says "Nonbuilding structures...
