I know that I've seen universal AV-cable to RFU adapters. Unfortunately, these would require the Wii standard AV cables to be plugged into a large brick which would then output RF. I know because I've actually been looking to buy one of these myself! Unfortunately, there ARE quite large and cost $20... but then again, they are UNIVERSAL, meaning they should work for anything that outputs AV or whatever, like a DVD player!
The one I saw was at a Walmart, or a Radio Shack, but I'm sure they're also online... hmm... gee, they ARE sorta rare online...