Plugin to lookup the ID of a previously imported entity from it's GUID

I've mostly used Feeds Tamper when I'm building a few importers which hook entities up together with references of various sorts and I've always been puzzled by the lack of a generic "convert this GUID to an ID" plugin.

My concern with writing this plugin today, for example, was creating OG group nodes and then creating users and assigning them to the groups. Using this plugin (followed by the "explode" plugin to turn the single value into an array) and assigning the result to the "Group membership" field and it just works.