Is it currently usable in anything other than GrayJay?

I wish I could use this with local videos.

Cloudstream uses it too: https://github.com/recloudstream/cloudstream/blob/master/app...

For local videos with GNOME Nautilus, there is https://gitlab.com/futo-org/fcast/-/tree/master/clients/naut...

Someone also made it possible to stream anything that yt-dlp can download https://git.sr.ht/~shironeko/fcast/tree/yt-dlp/item/clients/...

There's a terminal client at https://gitlab.com/futo-org/fcast/-/tree/master/clients/term... which supports local videos.

Thus far, nope.