You could also use a Sans Amp rackmount as an insert on the vocal pre channel strip. Less to fiddle with in terms of midi, but you'd still have the ability to use the presets (and if you did want to be able to scroll through the presets via midi, you can on the PSA-1). Of course, it is footswithable.
The Sans Amps does line level, so you won't have any of the impedence mismatch problems of a Rat or Boss pedal, and the tones are infinately more tweakable.
mic>preamp/ channel strip >insert sans amp and vocal processor in series>preamp/channel strip out to house.
It should work great.
That said, you might want to try running a low-bandwith mic like the bullet (or way better, the Copperphone) on it's own. You wouldn't need a guitar amp, just run the bullet to a di to the house desk, or the Copperphone like a regular second mic.
Check out the website for the Copperphone as I think this does a better job of getting that lo-fi gramophone / am radio sound that any processor or other mic. And it wont break the bank or compromise the main vocal chain in any way. Did I mention the mic looks cool as hell? You'd solve any feedback problems as well.
http://www.placidaudio.com/index.html
Frank