I am a new user to Plex (and a developer as well) - I am not sure whether any Lightroom support plugin is being worked on, but for what its worth here are some findings which might be helpful:
As you are probably aware Adobe Lightroom is using a SQLite database as "Catalog" file. The format thereof is off course not documented and likely to change between versions (however I don't expect fundamental changes to the most common tables such as image paths and maybe their ratings, comments and keywords). What's more you get only access to the DB when Lightroom is not running due to a lock being set, but in the context of Plex we can easily live with that.
Anyway, here's a useful link I found which shows a query which involves some typical tables:
http://forums.adobe.com/thread/792671
With AgLibraryRootFolder, AgLibraryFolder and AgLibraryFile you can assemble the full filepath to a given photo (see query). With the proper metainfo tables (to be figured out as an exercise for the reader ;)) it should be possible to find out the orientation (in case it has been changed within Lightroom) and adjust the orientation accordingly when displaying the photo in Plex.
Additionally keywords, ratings and comments could be extracted and... well, let your imagination lead you :)
Oh and yes, +1 vote from me (as a user) for Lightroom Catalog support :)
