The Grope & Assualt Room is only if you require a full secondary screening. An opt out pat down would usually be done in public. As for your rights, in a practical sense, you have none at the time, but many (in theory) after the fact or if you happen to get some TSO's who know what they're doing and care. Until the TSA/DHS gets severely slapped down in court they will continue on their merry little way.

I'd also suggest that rather than opting out actively (as in, telling them you're opting out) tell them that you can't go through the AIT for medical reasons. You might start getting a simple walk through and the occasional swab. It's all I get these days.