I have yet to find Smart TVs incorporate this feature. Maybe someday but for now you create the content you want with a single subtitle and wait for Smart TVs to get smarter.
I just bought myself the LG 50LB56 model and it plays movies fine through my USB. Problem is, I can't get the correct subtitles and audio tracks when there multiple ones in the file. It just auto selects one and I don't see any options to switch it. For certain movies they come with commentary embedded into the video(no separate file in the folder for it) and I don't want to hear that. Any tips on how to change this?