The HP ThinkPad Tablet 2 and the Fujitsu Q584 both can charge via USB. The TP has a very good keyboard, though it is not a true clamshell. It is being replaced by the TP Tablet 10, so it is a particularly good deal as low as $300 US if your needs are modest. We expect our first TP 10s in about 2 weeks, they do have a MS Surface like touch folio, but seem to use a non USB charger.

The Q584 does have an available clamshell keyboard, and it uses the newer quad core Baytrail Atom, spec'd with 4GB of ram it is pretty capable performer. IT has a proprietary charger but does also charge via USB.

Charging via USB as others have said is perfectly fine overnight, but don't expect 15 minutes between meetings on someone's phone charger to get you much.