Hi Macker,
MIR is a bit different than a traditional reverb. I fear the way of using it has to follow a different approach. An easier one, probably, since there is a single stage to care about.
I don't know if the technique you are describing is the same I had been using before switching to MIR. What I did was to create some layers of early-reflections, to create the illusion of close, middle, and far groups of instruments. Instruments were bussed to these layers, to give an approximate idea of distance from the listener and the back wall. Maybe this is the equivalent of what you call stage-reverb. Then, everything was sent to the main reverb halo, that may be what you describe as the main-space reverb.
With MIR you deal with a virtual space, where each instrument is sent to its final position. From there, MIR creates all the components of the reverb, giving an idea of position on stage and enveloping the instrument with the reverb's halo. What you can do is to set the length of the reverb (as if you were actioning absorbers), mix the dry and wet signals (as if you were mixing the spot and the room mics), add a second set of microphones (for surround), and equalize the instrument's and room sound.
The main character of the reverb is therefore due to the chosen room. In my case I've chosen the Teldex, the classic recording studio very often used to record classical music. It's an average-sized stage, with a very transparent sound. Not exactly the huge space that is the RAH, or a bigger film scoring like the Sony/MGM or the Synchron.
So, if I want a lusher reverb, I guess the best option is to select a different room, with a lusher sound. I'm trying now with the Steinhofkirche, a huge building modeled after Hagia Sophia, with 8 seconds of reverb. I've lowered it to 3 seconds, to make it more similar to the RAH. Different spaces, but maybe they can be in a similar class of reverberant spaces.
Paolo