Hey, I AM an electrical engineer--they never taught us this stuff. And I've done software my entire career so what I know is what I've learned on my own.
Anyway, my understanding has always been that if you cut the power between the VFD and the motor, you can damage the VFD. I suppose you could wire up contactors, etc to ensure one and only one motor is connected at a time? I've also heard someone talk about putting a plug on the thing so you could move it machine to machine. My understanding might be wrong too--wouldn't be the first time.
Joe, I'm assuming your machine has a non-standard motor or the motor was otherwise expensive to replace? If that were the case and I were in your situation I'd do exactly the same. I'd probably only choose the RPC for running multiple machines. For my jointer I turned down a static phase converter the seller had for $50 because I was sure I could find a motor for less. I'm seriously considering putting a VFD and a 3-phase motor on an old Duro benchtop 14" DP I have for the variable speed functionality.