I'm sure that such a hook could already be written for UploadWizard,
though ideally it'd be done properly with a type handler;
the transcoding effort, however, would mean that we would not store the
original, so there'd need to be a challenging community discussion about whether retaining the original was important.
The Internet Archive would store the original. Their mission is keeping originals available in perpetuity. Bidirectional links between the IA and commons pages could preserve the connection.
It'd also be a pretty fragile system compared to doing the transcoding
ourselves, without significant moral, ethical or legal gains as far as I can see,
Is doing it ourselves on the roadmap? IA has offered to do it now, providing a practical and temporal gain :)
But "inside it somewhere" isn't a great answer, I'm afraid.
A more specific answer would be nice, in case the people making use of such a hook are not core MW devs.
SJ