We now have tunes!
I was looking around at a few stores, trying to find something that'd work, and came across a Logitech mm32 portable speaker system. Seemed like with some custom brackets I could mount it right to the stoker's bars. It'd be a $50+ outlay, but I was going for it. Until....
I stopped into Dick's Sporting Goods, looking for something else entirely, when I stumbled across a "tent speaker". It's basically an all-weather flat panel amplified speaker thingy that works on AA batteries and has the 1/8" plug to connect with any audio source. It says on the package that it can be used as a tent speaker by slipping it into the speaker pockets of the Dick's house brand tents. Or it unfolds and you can sit it on a table. For $10, I figured I'd find a place for it on the bike.
And just as I thought, the thing fits on top of our rack-top bag, underneath the elastic cord on top of the bag, and it's black, so you don't even know its there because it blends in. Plug in the audio source, toss it in the bag, and ride off with the tunes going. It even sounds pretty good.
Sometimes you don't know what you're looking for until you find it.