I’ve been using bc7 since 2012, and even have a video codec using bc7 keyframes (we needed alpha videos, even have a patent for it). Our emphasis is not file size, but playback speed. Bc7 renders 20-40% faster than raw rgba32, and we overlay 5-7 videos on screen simultanously, so every boost helps.
We also have custom encoders for the custom video format.
HAP is a family of codecs for GPU accelerated decoding. HAP R uses BC7 frames.
There's support for various HAP codecs in ffmpeg, VLC, etc, but I think support for HAP R is lacking. However HAP, HAP Alpha, HAP Q and HAP Q Alpha have wide support. They use DXT1/DXT3/DXT5/RGTC afaik.
Compared to your implementation, their addition of BC7 is quite recent, yet they did have support for alpha channels for probably a decade.
We also have custom encoders for the custom video format.