Yeah, mc is the closest option--there's nothing with a GUI. It's a common request, but as yet hasn't been implemented (and from what I've seen, the devs don't seem very interested in implementing it).
Edit: OTOH,
Cloud Commander looks like a pretty slick option. It's basically a web version of mc, which runs under node.js. Looks like it should be pretty simple to get running in a jail, though it would then (obviously) only be usable for storage mounted to that jail. Maybe I'll play around with that a bit.