You could create a custom launcher/shortcut, that fires off Truecrypt first. This is assuming Linux, something similar could surely be done with other OSs.
If your shortcut was basically:
truecrypt & multibit
It would start Truecrypt first, and then without waiting for Truecrypt to complete, also start Multibit. That would give you a firm reminder to mount the drive/file/partition first.
In Linux, if you executed
truecrypt && multibit
it would wait until Truecrypt was exited with no errors and closed before it started Multibit. This is probably not what you want, though it would work if you didn't mind starting Truecrypt again to cleanly unmount the drive/file/partition.
This is assuming you can just run 'truecrypt' and 'multibit' from a command line. You may need to give it a full path, like /usr/bin/truecrypt (or whatever). Since Multibit is a java executable, something like this just now worked on mine, and it found my wallets fine.
I don't have Truecrypt on this machine, but assuming you can fire it off with the simple command 'truecrypt' the whole thing would be something like this:
truecrypt & java -jar /home/YOURUSERNAMEHERE/MultiBit-0.5.12/multibit-exe.jar
YMMV, use at your own risk, no warranties implied. But, that looks kinda cool, I think I'll try it. I use Truecrypt, just hadn't thought about stringing those together like that. (just came up with that for this post)...