I personally really like the concept. But for anyone using git they’d have to be aware there was a level of indirection every time they pushed, which is bound to cause trouble and potentially problems with GitHub maintaining.
What if your team operated on a private repo – but you could highlight branches which were “published” to a public repo? Your commits would be taken – message attached – and reproduced against the public version of your code by an organisation account. That way your team get to operate with freedom, but their work is still open to inspection.
I know other orgs do this by their own infrastructure – but having it available in GitHub would be a very clear and straight forward way of promoting transparency in organisations