I don't think there's an easy fix for it, especially since MPP is currently iframed, and the code will probably look much different by the time the new owner finishes rewriting it. For now, you can just override the client's note press code to always use maximum velocity (note volume), which is 1 (0.5 would be 50%).
Something like
MPP.press = function(note, vel) {
if(this.isConnected()) {
var vel = 1;
if(!this.noteBufferTime) {
this.noteBufferTime = Date.now();
this.noteBuffer.push({ n: note, v: vel });
} else {
this.noteBuffer.push({ d: Date.now() - this.noteBufferTime, n: note, v: vel });
}
}
}
try that.