The only thing you can bring back without permits easily are softies - I brought back a powder blue anthelia from New Orleans - declared and all - they customs officer didn't seem to care at all.
I brought it back in a small bag with 100 ml of SW in it, and a metal thermos. Took it out of the thermos for inspection (xray machine) and put it back right after. Made it alive, no problems
Although, bringing back SPS is probably more of an issue, and you need permits. although Oceanic Corals seems to get a lot of proze pieces in from the States - maybe they can get what you want?