How to install app from App Store again on Mountain Lion (or on the new OS X)?
I've run into this before, and there really isn't a perfectly clean way to do it. However - if I wanted to trick it...
If you unmounted the disk/partition that had the existing copy on it, then App Store couldn't find it. Have you tried renaming the existing copy, or temporarily moving it off the Lion partition to USB key, then rerunning App Store?