Thanks to the helpful people in the local TV extras thread, the extras added by the agent have been confirmed to work on Plex Media Player (for Show extras), Roku (Show and Season), iOS (Episode only), Plex for Kodi (Show and Season) and Android (Show, Season and Episode) in addition to the web client (Show extras only).
The official Apple TV client doesn’t support any TV extras (which is a disappointment to me), but a lot of other clients work on at least a minimal level.
There is already server support for extras to be linked to TV Shows, Seasons and Episodes – so my hope is that Plex plan to update their clients to display and play them at all three levels as the Android client currently does.
Although, if I had to pick one to update first, it would be to enable extras linked to seasons and episodes on the web client, since that is the experience common to everyone, plus it would make it so much easier to check if they’re being added correctly 
@elan if you’re listening?
