I was thinking of something like this myself for my shop

You can get a package of stereo headphones, with a small transmitter that broadcasts at the lower end(88) of the FM band, range about 2-300 yards. You can get them in the UK, with a sticker saying that you can't use them!, so you must be able to in the US. Novelty and cheapo shops might be more fruitful than hi-fi or electronics!

Obviously it would also work with a small portable, as you suggest.

...also dead handy if you're stuck with a hire car with no cassette slot/aux input and want to use your Empeg.

N.B. I forgot to mention that the stereo headphones you get in the same box, work perfectly well as an FM radio, ideal for the garden/a bit of privacy - the whole lot only cost around £20 - I'll check if they have a brand name.