Actually, I tried this sometime ago, before I've read this thread (just now).
The problem is, although the tremolo circuit seems to be placed after the return jacks (as per the manual's diagram) and thus should effect the incoming external preamp signal, there is no tremolo effect at all if the external preamp is 100% mixed in the MP2's return loop.
My guess is that the signal from the tremolo effect is itself mixed in parallel with a dry, non-tremolo effected, audio signal from the MP2 preamp section and thus mixing something 100% in the loop fully defeats the tremolo effect (maybe a question of switching sends vs switching returns).
Another consequence is that the only tremolo effect you hear is imposed upon the ADA preamplifier signal exclusively, not on any other external signal (I'm not near my MP2 right now, so can't fully testify this, but I seem to recall this).
Which, if this proves to be generally true (and not some specific bug affecting my two MP2's), is a real shame as you can't use the MP2 as an analog tremolo effects processor, with a very good sounding analog tremolo section that can't be used elsewhere (when apparently it would be easy to be used in such way given its placement in the circuit, post-effects loop).
Does anyone know if my guess is right regarding its behavior or if there is any other thing to it?
Or, am I the only unlucky bastard which MP2's exhibit this behavior? (well, at least other folk would be able to take advantage of the MP2's nice tremolo applied to other preamps)