Unfortunately, once you delete a movie from the HDD, the Lite-On recorder itself offers no method of retrieving it back.
However if you urgently need to recover a recording its HDD, you should be able to connect the HDD up to a PC and use any file undeletion tool to retrieve it. From what I remember reading before, the Lite-On uses FAT32 (or something compatible) to store its recordings as some have put the hard drive in their PC to backup recordings from it. If this is the case, any undeletion tool that works on FAT32 should be able to restore the recording, assuming you have not made any further recordings to HDD or performed disk defragmentation. :doh:
Just be warned that if you do decide to open the cover, you may end up voiding your warranty on your recorder. From what I can see, there are no visible seals on the recorder itself or its screws. In my Lite-On LVW-5045, there is some tape covering the cable from the IDE DVD writer, however this is not the case with the HDD and it looks like the HDD can be easily removed.