Very cool. Yes, IMO 1080p subs are the best move for compatibility. Right now, I treat 2160p subtitles as more of an archival format – useful for special purposes, but not for most direct applications.
Complain all I want about the state of Blu-ray subtitles, they are very cool in that they allow stuff like what you’ve just done. Old school DVD 3 regular colors + 1 full transparent color would look more like an encoding glitch than a reel change marker.
Other uses I’ve seen for non-text subtitles include alternate aspect ratios, where the underlying video is open matte, but with “black bar” subtitles, you can reimpose the matte and get a theatrical aspect ratio.