Actually gonk, all that I would need to be able to do as an interim solution is switch 5.1 channels. The Toshiba HD-XA2 only has 5.1 outputs, so I can just permanently plug in the surround back left and right channels from the Panasonic DMP-BD10 and only switch the remaining 5.1 channels.

It's really amazing to me that this problem is turning out to be so difficult to solve. It seems as though the consumer electronics industry has made a collective decision that switching multichannel high-definition surround sound will be accomplished solely by switching HDMI. That's great for the future, but what about legacy systems like most of us mere mortals have?