I kinda figured Phil was going for the novelty factor of an in-built amp/speaker combo, I mean, sure it might not work as well for bass as for guitar, but the guitars aren't serious instruments either, are they?
Really, I'd just go with a Smokey circuit based on the LM386, or the GuitarFetish speaker amp, and find the best efficiency small speaker with decent response down to 40Hz (the frequency of the low E on a bass), something like
this from Digikey, and call it good. You could increase the headroom to the circuit by using two 9V batteries, or use a charge pump like the MAX1044 which is cheap and designed for audio applications if you only want a single battery.
I've played around with some stuff like this and it can be some fun.