I started working on this last year with a set of rx8 housings i have. A different purpose than yours though.
Its pretty easy to talk about welding the coolant passages. But a different story to actually do the welding. (I am a welder) You would have to do TIG to get a clean weld that would be strong enough. And its almost impossible to get the TIG torch down in there close enough for good penetration. You can get a MIG gun in there and try to spray weld it, but you will not get good gas coverage. But this is cast aluminum, and even with billet aluminum with good gas coverage, spray/MIG welding will not be strong enough for this situation. And the heat needed to weld such a piece, will warp the housings.
So I'm looking for other ways now.
Another way that i wanted to try is similar to a PP on the intake. Press fit (extreme press fit) a sleeve for the exhaust. I have heard of a few people machining exhaust sleeves from aluminum and them working fine. I don't see how, as i would think that the aluminum would melt.
Then after a lot of searching, i found this...
http://jhbperformance.com/downloads/...racingmods.pdf