Ok, several years ago I took the lower unit off an outboard and shipped it off to a shop to repair the skeg. They said there was no reason to open the gear case and remove the guts before sending it off. That it was just extra unnecessary work.
Does aluminum dissapate heat so quickly that the rubber gearcase seals don't get warm enough to be damaged? Do they maybe wrap the gear case in wet rags while welding the skeg to keep the seals cool? I wonder.
I know that HVAC guys will often wrap a valve with wet rags to protect its guts while soldering a refrigerant line into the unit, but while a torch has to get pretty darn hot to melt silver solder, I imagine its nowhere near as hot as the metal at the point of an arc when mig or tig welding.