It may or may not be doable as a software-only solution. My guess is that they're already dealing with a limited power and hardware budget, and adding the decoding of a second data stream plus the combining of outputs may be more than they can handle in existing products. Plus they might need a dedicated button on the remote to make switching between sources quick and easy; I really don't want to have to pile more stuff into the * button's functionality.
From a marketing standpoint, this would be a major new feature and a huge selling point for new hardware. I've gone through multiple generations of Roku hardware - each adding some new capability and/or performance - so I don't imagine this would be any different.
As for compensation, I'd feel more than rewarded if something we post gets picked up as a feature in some future product. After all, the reason we're posting ideas is because we want to see them in our living room!