I've heard talk here and there about "loop points" in wav files that Q2 uses. I don't have a clue how they're implemented though. There's no loop-point paramater for PCM audio in the RIFF WAV specification (that's what Q2 uses as far as I know) so whatever it is it must be non-standard. I don't recall ever seeing any information on it on any sites anywhere. Somebody familiar with the Q2 source should probably dig around and find out what the deal is.
Edit: Oh, to answer your other question, try setting the target_speaker(s) to looped_off, have a trigger_relay turn it on, and a second trigger_relay with a delay on it turn it off.