@sspeed - you can see an MPL by creating one. Select a couple of files, and do a File > Export Playlist, and select the MPL format, and select the Output Range: Selection option at the bottom.
We need to be concerned about the import process via MPL in your case, since you don't have backing files. Each file-less import you do will create duplicate entries in MC (since MC uses the Filepath as the unique per-file external identifier), whereas if you have dummy files you can repeat the import time and again, and this will only update the tag values.
@raldo - probably your best bet would be to export an MPL of the movies you want to delete, and then remove the Field=Filename entry from each Item inside the playlist, and then you can delete the MC items and do a File > Import Playlist of the MPLs. Now, you'll have file-less entries.
Enough folks have wanted to import data into MC via a spreadsheet, that I'm thinking it might be worth making a general conversion, dummy file creation, and import script. I can cobble pieces from my amg.pl and discogs.pl scripts as all the guts are in place (CSV conversion, MPL creation and/or automatic updates via MCWS, and field mapping). This would allow you to setup your Field mappings (your spreadsheet column name to a stock or custom MC field)