Mike, unless he wants something more complex than your description implies, the simplest thing is to just treat this as a variation of the all-channel stereo mode. Keep in mind that in that mode the left front channel is duplicated in the left side and back surrounds and likewise for the right side(if a center speaker is present, it gets the combined L/R channels). So, using any 7.1 receiver that offers that mode(most or all)the ceiling speakers(not enthusiastic about that placement)would use the front and side surround channels, while the deck speakers would be connected to the back surround terminals. When the deck speakers weren't needed, the back surrounds would be set to "None" on the remote. The usual suspects have fine receivers from about $200 up that'll do the job.
Another possibility is to use a receiver(e.g. some Onkyos)that has an A/B provision that uses the back surround channels for the B speakers, instead of just being a parallel connection to the front channels. The A/B switching would then be used, of course, to select the speakers in use.
