You're right, it doesn't seem to be in there--strange that hasn;t come up before because I think it was added in v2004 (it used to be 20 stimuli per trial limit before that!).

May I ask why you want these to be all on the same trial? Note that back-to-back trials within a DirectRT session is just fine. It's running back-to-back sessions (or input files) without an intervening item (like an MediaLab instruction screen) that can acause trouble. A bunch of DirectRT sessions with 500 trials a piece could be scattered about your MediaLab questionnaires, no problem.