Hi,
Here in the UK, Humax are popular STB's used to access either Freeview ( DVB-T / DVB-T2) or Freesat ( DVB-S2) channels, with integrated add ons for all of the On-Demand content available from channels featured on Freeview/ Freesat.
Humax have a " App Market" ( https://www.myhumax.net/AppMarket) which features a limited range of apps which can be installed on the STB's. I believe they are HTML5 based, and if I remember correctly the Plex for Samsung app uses a HTML5 based browser to act as a Plex client.
Would this be a possible route to bring Plex to Humax?! It would be a very popular feature as Humax boxes are popular here in the UK, and also have very good review's!
Alex