I believe a mrss feed shouldn't be too hard. You can register either an external plugin event look (like index.php?/plugin/mrss-feed, using hook 'external_plugin').
Or you could create your own RSS Feed version by creating an event plugin that listens on 'frontend_rss', modifies $eventData['template_file'] to point to a plugin-file like 'mrss.tpl' which uses the appropriate nomenclalture defined by Yahoo. The data itself needs to come from the plugin, which can iterate over the media database, or on recent entries using preg_match and extracing <img> sources.
Basically, this can either be developed on its own, or patched into the greybox plugin. However, I think this media-rss thing is a feature to stand on its own, so I'd rather suggest to create an own plugin for that...
Personally, I don't find this piclens thing very convincing - but I can understand that people do. It's however that I personally cannot offer to implement that plugin. But I'm willing to help anybody along the way
Regards,
Garvin