Hey, it's certainly a cool idea, but as far as I know, this isn't currently possible over Lightning/USB. You could of course route audio out of your Mac's interface to/from the iPad to process (if the iPad also had an interface), but latency will be an issue there. I doubt you'd get noticeable enough performance gains for this to be worth doing.
I'd just stick to the desktop and buy another axe, or use your iPad to record/jot down ideas while on the go, and then transfer them to your desktop.
