The M Audio EX-P actually works !!! Don't throw it out just yet !
After buying this expression pedal without actually checking if it's compatible with my Line6 M5 or not (I know, not too smart), I was a bit sad to find out it was not fully compatible. For me it kind of worked - I could get it to go all the way up but I had to flip the pedal (so heel was the effect all the way up and toe was the effect all the way down) or it just wouldn't go fully wet or fully dry for other effects.
I read a few forum posts about this and in them I saw a couple of comments here and there that said exactly what I'm about to say : if you remove the 1k resistor and add a wire in its place it works properly !!!
Yes, you do have to open the pedal up, desolder the resistor (it's really tiny and located on the potentiometer circuit) and solder a wire in its place. Even though the resistor is tiny, the process of desoldering it is fairly easy if you have soldered before and if not, just watch a quick youtube video on how to do it. I think for the price (I got it for 14 GBP) it's worth trying out this mod. Wish I would have taken a picture but was to enthusiastic to try it out and forgot. I will add some if somebody needs instructions.
Hope this helps !