It wouldn't be too hard for someone with the ability to weld (preferably aluminum), but I think I'd much rather modify the existing manifold - weld the ports down to a round shape - or just make an adapter plate.  I compared the two gaskets today to see what is the same and what is different.  I don't have a 22RE manifold to look at right now, but as long as coolant flow and volume through the tall rectangular outlet betwee ports 1 and 2 is not absolutely critical, I don't see any reason this wouldn't work.

http://public.fotki.com/te51levin/manifold-gaskets/

The red area is the port shape difference between 20R and 22R.  Not too radical, really, and practically zero at the roof and floor.