Hi Adrien,
I assume you are talking about MIR Pro, aren't you?
You're absolutely right: We record impulse responses for MIR Pro's Venues in empty halls. It wouldn't be easy to find up to 1,600 people sitting completely motionless and silent for 24 hours or more, listening to loud sine sweeps all the time, just to fill the audience area of Vienna Konzerthaus - Great Hall or Sage Gateshead - Hall One, that's true. :-) And even then, any Venue license would cost at least ten times what it does now. >8-0
But the good news is that good concert halls take this problem into account by padding the underside of their unused chairs with sound-absorbing materials, so the effect is much less obvious than you might think*). The acoustics of churches and the like will not change noticeably with or without people in them anyway. And in the case of studios and scoring stages, you wouldn't expect many people to be present during a recording session anyway.
You can still easily imitate the most noticeable sound-absorbing impact of the audience by reducing the reverb length a bit; you can also dampen the high end of the wet signal a notch with MIR Pro's RoomEQ.
If you are aiming for a completely different sound impression without losing the sense of depth of a selected Venue, try a hybrid approach by adding algorithmic reverb to MIR's Wet signal (or even more drastically: by replacing the actual reverb tail altogether and keeping only the first 300-500ms of the IRs).
*) Apart from that: I know conductors and record producers who prefer to hear the sound of certain halls empty than filled with people. 8-)
HTH,