Yeah, I am suspecting it isn't possible. DVD-Lab seems to let you specify what each language is, but apparently not what each "mix" is.
And, the reason that my DVD player is saying that both are stereo, is presumably because I encoded the mono mix as a stereo track with two identical channels. The reason I've been doing it that way, is because apparently some 5.1 systems play back mono in the center speaker only. Since many 5.1 systems have a center speaker that is inferior to the stereo speakers, I encoded it as stereo so that both mixes will come out of the main L/R speakers.
So now here is my next question... supposing that what I have said above is true, and the DVD player is capable of detecting the nature of the audio source and display that (when selecting audio on the remote), should I re-encode the mono mix as mono AC3 instead of stereo AC3? It seems to me that on the "plus" side, the type of audio would be properly displayed. On the "minus" side, some systems would play the mono mix through the center channel only, making it sound (on some systems) more anemic than it really is. As of now, I'm leaning towards leaving the mono track as stereo/fake mono.
Thoughts? PSB is done, I just have this one last decision to make.