This jogged a memory, and after a bit of digging:
Don’t know how I forgot this one. It was implemented in Europe, Channel 4 used to broadcast films in this format mid to late '90s. Not HD, but at least it was proper 16:9 with the right decoder. Instead of cutting off sides, it letterboxed the 16:9 source, downscaling vertically, for display on ordinary 4:3 sets. Then it ‘hid’ extra information in the black bars (coded as chroma only), which a decoder could use to reconstitute the original vertical resolution for fullscreen 16:9 display. Ingenious!
