The very latest Mission pedal appears to work with H9, and it has the nice feature that you can plug a switch into it and connect a TRS cable to H9.
The older ones are really volume pedals rather than expression, so to avoid the dread 0-100-0, you'll need to snip the ring wire off the jack.
We like the Ernie Ball. I've changed the string in five minutes (but you need their kit).
