I have now tried this. I am using a Denon 3802.
I had to set the front L&R to large so that I could then set the surrouns to large, this was so I would get all the frequecy at the rear otherwise the sub did nothing.
I also set the Sub to LFE. The choice is LFE or LFE + Main. My understanding is that set to LFE the lows still only go to the sub for the front L&R and if I set it to LFE + Main the lows go to the sub and the L&R. Not sure if this is correct, the manual does not realy clarify this, so if someone knows better?
I then tested the opening to Toy story 2. To check it had worked I switched the front sub off so that I was sure the rear one was working and sure enough I get sub bass only when the rear effects work.
Then I reconnected the front sub and did a further test and all seems fine.
The best bit to check is when Buz getting blown into the sky when the robots blow up. Just as he lands the first piece of robot flies from left to right in the surround, what a difference, get this grate swoosh with a nice bit of bass as the piece fies left to right and then lands.
I hope this makes sence.
Lynx. The flaw in your idea is that I would get all the sub bass going to the rear sub which would defeat to object and would also sound a tad confusing.