ASCE 07-05 is silent on an enclosed building elevated on a frame (e.g. a beach house on pilings). Is there any guidance for the load on the under (floor) side? Is it reasonalbe to assume it is the opposite of the roof load?

Don't think you would assume opposite of roof load. Underside of structure would at best be assumed as a "flat" roof but it is at a lower elevation then typical roof plane and therefore the analysis could be time consuming to obtain realistic values; and some of that load would be in suction and therefore resist uplift... I'm not aware of any provisions for calculating such loads in ASCE 7 or IBC;