Is there a way of "re-locking" an internal FileVault 2 encrypted partition?
Due to the limitations, you might want to use an encrypted disk image rather than whole disk encryption.

Come to think of it - why not use both? 

You could always turn off FileVault if you felt it too much a hassle, but to get around the limitation you caught (thank goodness you did catch it),  consider using Disk Utility to make secure the parts of your data you need to force into a locked state when that disk image is unmounted. This will let you sleep your mac and not compromise that data by allowing it to be re-mounted without the appropriate passcode being supplied.