Damn, this thread turned hilarious fast.
I see your point but welding with a tig torch and having you a/c balance set for higher cleaning action, cleaning with chemicals will not remove enuff of the oxides and will require higher cleaning action dialed into the machine because of the condition of the metal means you will put a conciderable amount of heat in the base metal and it will heat soak just like brazing if there is rubber seals that close that you are worried about the heat then disassemble the trans and do either process.
please explain why you would not use brass or aluminum?
edit: you will have to pre-heat the base metal (cast metals expand and contract differently than the filler metals used) so ether method you will have to use a torch to heat the base metal to about 300-350*F. if it were my truck needing the repair i would braze it just because of the characteristics of welding cast aluminum.
I'll do my best to answer the post made in response to my previous one.
I'd use chemicals to degrease as much as possible.
You can play with the AC balance however you like, it's only going to help remove the aluminum oxide on the surface and won't do anything beneficial in removing the greasy dirt.
A file, sandpaper, burr ... etc can also be used to help remove the oxide layer minimizing the use of AC balance.
Brass isn't used to braze aluminum, it just doesn't stick very well. There are other filler metals that can be used which are more appropriate. For example, 4047 can be used as a brazing filler.
As far as a filler metal for welding it, there are also many choices. I'm sure one could be found which is suitable for this application.
With a suitable filler and welding procedure the weld and the untouched casting would have similar strength charactericts, assuming the part wasn't originaly heat treated.
A big part of doing this repair with welding or brazing is the skill/experience of the weldor performing the work. Also, the equipment and supplies they have available.
In summary ...
The method of repair is the customers discretion.
My personal preference would be TIG over MIG but I am also not against trying a quality epoxy first. JB Weld probably wouldn't be my first choice.