Rails is good for that. Worry about 'bloat' later.
> Second, how does one learn enough to hack something quickly together in the framework you suggest?
Start hacking on it, keeping in mind that you may throw it away and redo it.
Actually, I don't know if it is true, but I wasn't happy with Rails the last time around, and I want to learn something different.
If you really wanted to try something 'out there' you could fiddle around with Erlang, but that's probably more fiddling for fiddling's sake than 'getting stuff done'. I think for most people, Ruby, Python, and others like them are where the sweet spot is in terms of productivity.