Hey, mruvio:
I own the '88 too, and have the same question... Given the fact that the Piano V does accept continuous data for the sustain pedal (and this is important to a realistic piano performance), I think that the Sustain input should accept this kind of data.
By the moment, my solution is to use my sustain pedal (that sends continuous data, not only on/off) connected at the Expression Pedal in as you say, but It leaves me without an expressions pedal for other uses...
Be care with it: if you have a continuous pedal, and plug it into the Sustain port, maybe you have to invert the max and min (I had to)!!
If someone knows another solution, I'll be happy to read about it!!