I had figured that an external identifier might be needed by more than one application, so was a potential addition to the database.
The plugin_storage isn't really that useful for this need; it would require the plugin to maintain a separate data structure (and all of the managing routines). This seems like a really high overhead for something that could prove beneficial to many other plugins, or could if they knew one was there. ;}
As a programmer, it also feels to me like something that got left out, because nobody thought integrating with external programs was important. That may not be the reason, but when you have the abilities to install plugins that potentially automate from other sources, then some additions for ease of integration should be considered. A way to uniquely identify an image is such a one...
Anyway, it would be a simple and useful expansion to the whole idea of automating other sources, and does help immensely with the problem of cross-indexing images between zenphoto and external applications. Thus the request...
Cheers!