Technically you can use XNA too, and you can make a browser using that. Ofcourse that means you have to toss away all the platforms goods and libraries you have and produce that on your own as well. Not really the sexy option, no.
So not saying it's going to happen, but technically speaking it's not impossible either.
Sure it is possible theoretically but not practically. If you want alternative browser for WP7 you need to create engine from scratch. You could not create browser that can compete with current 4 engines for one or two years.
Quite possibly that we've never see new independent browser engine.