It’s for sure possible, but it needs to be implemented separately and won’t share too much code. And each submission service has its own quirks and specialities to take care of. Hence I can fully understand marc not being interested in implementing and maintaining it. I wouldn’t be either.
Regarding MIT licensing I think it is a good choice. MIT puts less restrictions on the code, the code could even be used in closed source. From a purely user perspective it makes no real difference.
I’m not even sure using GPL for a foobar component is really license compliant, as foobar itself is closed source. But IANAL, haven’t checked the exact licensing conditions of the foobar SDK and honestly also not really interested in getting into this in depth 