One issue could be that many of the devices which you would want to power, have their USB connectors hidden behind rubber, waterproof covers. Having them plugged into a power connector, whilst in use, will negate the waterproofing quite seriously.
This is something that annoys me immensely about the USB eTrexes. The older serial versions have a proprietary connector which consists of four gold-plated contacts flush with the outside of the case, which, while still protected by a rubber flap, seem pretty decently waterproof. Since the connector slides over the top of them, held in place by the shape of the case itself, this gives extra protection, and I'd be reasonably confident about using the thing in heavy rain. The mini-USB connector is a massive step backwards in this respect, as it forms a nice channel for water to get inside the case.
Adding a suitably shaped rubber flange to a mini-USB cable might work well enough for an eTrex, as it would tuck in under the open cover, but I wouldn't want to try that approach on devices with more exposed connectors. I think the pragmatic solution is to run on battery power when it's wet, and have the dynamo charging a spare battery in a nice dry bag somewhere. Which is clearly annoying for most phones, and impossible for certain popular Apple products...
Inductive coupling is a nice idea, but waterproof electrical connectors are a solved problem. And it's not like the devices we want to use have either...