Hmm. Based on the responses so far, I think the best option would be a combination of:

1) Amend fstab to include /dev/sdb1 as well, so that mounting from the command line is easy.

2) Write a script which will unmount /dev/sda1, remove the USB module from the kernel and reload it... thus effectively 'rebooting' the USB storage system and ensuring that any USB memory mounted subsequently is /dev/sda1 again. That will ensure that I can still mount from the Fluxbox menu, if I'm too lazy to open an xterm. Heh.

Not necessarily the most elegant solution, but if it works without breaking anything, that's good enough for me.

Many thanks for all responses.