grand piano pedals with TR-88

[vanagon]

I am trying to get a standard grand piano 3-pedal configuration for a tr-88 (soft pedal, sustenuto pedal, damper pedal).

I have two m-audio SP-2 pedals that I am currently using as the sustenuto pedal (in the "Assignable switch" jack) and the damper pedal (in the damper jack).

That leaves the "assignable pedal" jack. So what I want is a way of getting a pedal in that jack to behave like an on/off soft pedal like on a grand piano. (Or, possibly, some other solution I haven't thought of).

It doesn't seem the SP-2's work. I think the jack is expecting a continuous signal. But I would want a pedal capable of working with this jack which would still only have two levels so that it could behave like a standard soft pedal.