Good point Dru,
I guess you can tie another prussik above the knot, slowly weight the system, then tie a Alpine butterfly below the knot as a backup, clip into that with your regular back up, finally with two backups, switch the device, installing it above the butterfly so you can untie it.
At work we use our jumars in place of the prussiks to pass knots.
It's hard to have much redundancy with only one rope.