You will be replacing anything that's battery powered every couple hours if it actually exists.
A better solution dollar for dollar would be another dock that includes speakers. That's what we did in my daughter's room once the older iPods were upgraded. You can often find another dock on sale if you peruse ebay/amazon/froogle. I scored a memorex for $20 and a Logitech that was on closeout for $30. These work wonderfully, although I realize you are looking for wireless.
Hi, I have a mp3 dock in my den and I'd like to play/broadcast the music into my kids' rooms (about 50 feet away). Are there any recommendations or starting points for finding wall mountable speakers that do not require an AC plug (ie. powered by speaker wire? or possibly USB? I'm not sure what options are out there).

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic