Sure

> Do we really want to see non-MIT fixes in Mantis? :)

No
Now is it "legal" to say that?

> I think, Mantis should state exclusively at front page, that all
> contributions are going under MIT. If people don't want their code to
> be released under it, they can either release as separate package or
> don't release it at all :

I think that each package should have a license clearly stated as part  
of its documentation.

Stef