Currently only one CA provides paid certificates with a special extension so that the cert can be used to sign SXG files [0].
[0]: https://www.digicert.com/account/ietf/http-signed-exchange.p...
As for binary transparency it's not enough to stamp the certificate (that's what CT logs do). The artifact would have to be stamped and published in a widely accessible source. Actually Binary Transparency doc published by Mozilla [1] creates a new regular certificate for new published binary thus utilizing CT infrastructure as it is today.
[1]: https://wiki.mozilla.org/Security/Binary_Transparency
If we're at Mozilla, it's also interesting to see what's their position on SXG [2]. There is only one spec there with that status there.
It seems the real issue at the moment is that it just isn't a high priority for them.
Displays the origin of the content?
This may be useful for improving security, especially of CDNs. Binary Transparency seems to be one of the use cases mentioned in the spec[1] - perhaps someday this would be used for an unified scheme for signing application packages/updates, without reinventing the wheel every time.
[0]: https://developers.google.com/web/updates/2019/03/nic73#sgx
[1]: https://wicg.github.io/webpackage/draft-yasskin-http-origin-...