This feature already exists.
All that was needed was for me to create a resources\images path under my calibre configuration directory and move the plugin thumbnails there.
Recently the ability to create GUI plugins was added to calibre. Many of these plugins add thumbnails to the Calibre2\resources\images folder. This works great but has one drawback. Previously all plugins, tweaks, and settings were stored in the calibre config directory. This meant I could run calibre from any computer and point to this configuration and have the exact same setup from every computer. Now the thumbnail images are hanging out in a separate spot and have to be accounted for or updated on each computer.
I don't know how feasible this is but it would be convenient if there was a image area in the config directory where the added plugins could stash their thumbnails. Thus keeping all updated configuration files in the configuration directory.
If this is something worth doing I'll be glad to create the ticket (most I can do given my programming knowledge is nil).