I would try a 1010 Music BitBox Mk2 or BitBox Micro. They offer clockable looping. So when you reach a point where you're ready to transition, you can sample a loop of your outgoing mix, crossfade between the two, then set-up your new patch as the loop does its thing.
You may want to do this OUTSIDE of the rack. I'm thinking the 1010 Music Blackbox and a DJ mixer with crossfader. The Blackbox won't take up an HP as its an independent device. It's also clockable and loopable like the Bitbox series. Also, having it independent of your rack will make life a lot easier especially if you're trying to troubleshoot. You could not only loop your outgoing mix but also have something entertaining on standby in case everything all goes to hell and you need 3 minutes to figure things out.