It is pretty hard to weld with the pan on usually. The type of welding necessary would be TIG welding.
TIG, Tungsten - Inert Gas welding is sort of like torch welding. The concept is to be able to apply heat untill the subjects are liquid and then connect the puddle, maybe with some filler (of similar material - welding rod), and let it solidify as once piece.
The point with the Tungsten is to have a point source of high energy current that is not consumed. The arc brings the heat, like a torch. The point of the inert gas is that aluminum burns at the temperature that it melts at. The oxides form barriers to a uniform puddle if only in small amounts. Here is where the problem comes with the pan being installed. The oxidants aren't just the oxygen from the air. If any oil remains in the crack the oil acts as the oxidant and the inert gas can not suppress it. Ask how i know this.
It can be done if the pan is washed thoroughly and drained and then heated first with a torch till all the oil is gone in the area of the crack. Of course I'm not sure how to wash the pan without using something likely to explode when heated with the torch.
The plastic metal will be OK if you find a product that doesn't fall off someday while you are driving down the road. I have used a product called Aradite (sp?) made by Ceiba- Geigy (sp?). A friend introduced it to me. He first used it to fix a porosity that was happening to GM automatic transmission cases. It was a factory service procedure or bulletin in the early 80's.
Also, don't expect the plastic to do any structural portion. It can be used to stop leaks, but I would not consider it safe for a structural patch.
Bosch Master, ASE Master, L1
33 years MB technician