Hash of an item as GUID

I'm importing items that don't have GUIDs. On import, I want to ignore the item if an identical item has ever been imported previously, and create a new node if no existing identical one exists.

I've come up with a workaround to accomplish this that calculates a hash over all of an item's yet-processed source results, and stores it in the $variables[] array as a sort of special replacement variable. (I'll attach the patch in a comment.) I then map this variable as the feed item's GUID.

I'm sure there are much better ways to do this, and would appreciate advice in that direction. It looks like Feeds has its own built-in item hashing, but it's not obvious to me how to use that to identify and act on uniqueness vs. duplicates, and it appears that feed items require a single, uniquely-identifying field.