@Grumpy_Mike - thanks for checking for me! I'll buy the bits and have a play.
@retrolefty - Yup, a relay was my first thought, but I've only got one 5v/latching one knocking about and I want to save it for bigger badder things. Thought it would be a useful exercise to try and keep the whole thing solid-state now I'm getting a little more experienced with all the analogue 'glue' needed to have my AVRs talk to the outside world.
[edit:] Just looked at your schematic; never come across single coil latching relays before, I've only got double-coil ones. Interesting. A bit confused about C1, though; why is it there? Doesn't it stop the current flowing before the relay's even had a chance to think about moving?