I have an xr18 and I know that there are midi CC's for fader functions.
FX can indeed be controlled by MIDI. I use a laptop with Showbuddy to fire Snippets so I can't think of any reason that a controller would not work. I am about to try the same thing with an iPad app.
A Snippet is a 'state', almost similar to scenes on the Axe, so in one snippet I could set one type of reverb on the lead vocalist and another on a backing vocal and store it. I will say the X32 is a MIDI hog taking up 6 channels in the following format:
Ch 1 - Scenes
Ch 2 - Snippets
Ch3 thru Ch 6 - Cues
And the channels are not changeable.