I Jumpseated from San Juan to St. Thomas last year on Caribbean Sun. I went to the gate and asked the agent if they allowed jumpseaters from other airlines. I filled out the jumpseat form while she made a quick call and within a couple of minutes I had my boarding pass. I think the agent made a call to operations or dispatch to make sure that there were no issues with me jumpseating. When I made it out to the airplane, the pilots and flight attendant were absolutely great. They were more than happy to have me on-board. The whole experience was actually much easier than trying to jumpseat on an offline carrier here in the States!!
(Not sure on how it works for international jumpseating from SJU, but going to the US Virgin islands was no problem).