The reverse camera is a simple composite video signal (just like the old "RCA" video cables you'd run from your VCR) - so your 'new' video source will need to be able to send out that sort of signal. You'll have to wire in a relay to switch the input from the backup camera to the source you're trying to use in addition to the kit above, meaning the composite video signal wire will need to be interrupted - but if that kit is a pass-through of the 40-pin AV-nav harness (which is what it looks like), you can make that modification within the wires of the kit itself, so still no car-wire-splicing needed.
*edit*, you're right, that wouldn't take care of the reverse guides. Not sure of a good way to do that, or if it's even possible. I believe the steering wheel position is transmitted to the avnav via canbus, and disconnecting can signal would cause too many other problems to be feasible.