The Ringmods VCA/volume pedal functionality seems to operate at a -2.5v to +2.5v range (instead of the standard 0 to +5v of other inputs), so if in Unipolar mode you'll still only be giving it 0v when the Heel is down.
When using expression mode, with the MP201 set to unipolar (0/+5v), the full volume is met at the halfway mark as you noticed (about 2048 at toe down) and then goes back to silent at toe down so it works more like a manual tremolo, and there is still a little volume leaking through when the heel is all the way down at 0v. If you have a CP251, you can use the Unipolar expression mode with the full 0 to 5v, by using the CP251's attenuator and setting it to -2.5v.
I haven't tried this, but I think you can achieve the same result using only the MP201:
*set the MP201 to Bipolar mode in the global options menu, so it is now operating in a -5v to +5v range from heel to toe.
*then in expression mode, try adding about 25% to the Heel-down value (bring it up from 0 to about 1024), and do the same to the Toe-down value (bring it down from 4095 to about 3070).
*doing this should have the expression sweep operating at about -2.5v and complete silence when the Heel is down, and about +2.5v and full volume when the Toe is down.
Let me know if this works out for you (i really think it should), or if I get around to it first I'll update with my results.
Synth bass guitar rig including MS-20 filter clone, Paradox TZF, MF101, 102, 103, 107(x2), CP-251 and MP-201