The plan is to make xyz.wikiproject.org redirect to incubator.wikimedia.org/wiki/Wx/xyz if the wiki does not exist. Preferably *all* language codes, but if that is not possible then only languages known to Wikimedia.
(So for example af.wikinews.org -> incubator.wikimedia.org/wiki/Wn/af)
The new welcome template will soon be deployed to incubator (hopefully).
After talking with Danny_B, Ryan Lane and Rob Halsell about this, it is difficult but not impossible. Apparently we should wait until virtual hosts are set up for Wikimedia.
It would be very useful if we could define the redirects ourselves, which will apparently be possible in the future.