I don't know if this is the kind of solution you are looking for, but there are many external switchers available. The Radioshack 15-1976 ($39.99) has 4 inputs and 1 output and claims to convert composite to S-Video as well, meaning that you could hook up the old VCR too and never have to change inputs on the TV. The Recoton SVS1000 (29.99) is similar, except it does not claim to convert composite to S-video.
I know a Recoton RF modulator is able to switch sources automatically (i.e. it passes 75 ohm through, unless the DVD player is on) but that wouldn't be the exact solution you want. But if their video switchers also have some sort of precedence, then you might not need to push buttons. Otherwise, you would have to manually switch to whatever input you wanted. Radioshack does have switchers that use remote control, but they are quite expensive.
If it is a short term solution, the Recoton sounds to be the least expensive solution and is available at your typical Best Buy/Circuit City stores most likely. I can't speak to the quality of these devices, however, so it may involve a return if it is not up to your standards.
Hope it helps.
S.