While debugging something unrelated, I was poking around a logcat log file from my Shield TV this morning and saw this line:
11-04 08:46:19.159 1854 2649 I Kepler Server: Addtional Plug-in directory: /storage/emulated/0/Plex Media Server/Plug-Ins.
/storage/emulated/0/ is what /sdcard points to and is user accessible and write-able.
If /storage/emulated/0/Plex Media Server/Plug-Ins is indeed an additional plug-in directory as the log line states, in theory we should be able to drop 3rd party plug-ins into this directory and it should work.
I'll have to test this when I get home but if anyone else wants to give it a try, let us know what happens.
Also, I'm also running the DVR beta 2 version of Plex Server so I'm not sure if this is something they tossed into that version of if this exists in the normal, non-beta version from the Play Store.