Hi again,
hopefully you won't ignore my proposals here again
XviD has nothing which would be close to nandub's motion detection engine.
Curve aggression hasn't changed and thus determines
a) how fast overflow gets compensated (i wrote that earlier already!) and
b) due to a cosine/sine bias of the curve, whether you prefer huge frame higher quantized than small frames or vice versa.
From my experience a) is much more "visible" effect than b).
So it's best to leave that at medium aggression. If you get problems with your quantizer distribution, you can try fidling around with that, but that's not easy to see before having done a second pass.
Since you get the most constant quality using a linear scaled curve I'd suggest not to fiddle around with altCC settings too much, as they can do real harm to your encode when setting impropper values.
If you find a nice high/low value that's ok. You could set a min. req. quality or the strength so that you have a "smooth" high/lowpass behaviour.
Everything else sounds like an adventure to me
Regards,
Koepi