Well, I tried it, and my toothbrush is happily charging away. It's been plugged in for almost 2 hours now, and thankfully nothing seems to be getting overheated or even warm.
It lets off a light buzzing sound, though, which apparently is "normal" or at least common ( newcomer link: http://www.diynot.com/forums/viewtopic.php?t=138077 [nonactive] ) in the average bathroom socket.
However, it doesn't seem like a long-term solution. In the long run, the socket could get warped due to overheating ( newcomer link: http://www.theiet.org/forums/forum/messageview.cfm?catid=205&threadid=27051 [nonactive] . Apparently, these are only designed to for the 10 minute shave (not all-day charging).
Anyhow, the loo socket seems a good short-term solution for low wattage US appliances! Thanks again for your help :-)