The reason they want to weld the unused holes up, is that they want to restore the original designed amount of material around the fastening points. While this is a novel idea, you might want to go and read up about welding aluminium and it's alloys, to see what a mission it is to do properly.
You have all kinds of problems with different metallurgical properties and mixtures of different metals to get the desired properties for a given application. I doubt that these materials are sold in rolls of welding wire and/or sticks, so your going to compromise with some filler that is not the same metallurgically, and you're going to fuse them together. They have different cooling rates, and the original metal's properties changes when it is heated to the point of melting, and you end up with microscopic cracks and faults, which is not something you want in a wheel.
Not even in a wheel that's going to see limited duty. Read up about it and maybe you'll change your mind, and possibly save a few lives.
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It would involve using an alloy dowel and tack welding it to the wheel once it has been really pressed in.
Logic tells me this will only weaken the wheel even further, because you are now pressing a dowel into a hole "to small" for it. This pressure is sitting there waiting to be relieved, and the moment you drill the new holes, and even if the have been pre-drilled, the pressure will by diverted towards there, as these are the closest relieve points. You once again end up with hairline fractures and/or faults, which is not good.
Just my 2c, as I wouldn't even have though of re-drilling alloy wheels. Steel wheels is a different beast altogether though.