Emailed KVP after the gunsmiths at The Range said they couldn't do it and he replied: My guess is that your Gunsmith doesn't have the equipment to weld AL? I know that the gunsmiths at New Frontier Armory pin and weld them quite often.
So it seems doable.
Thanks for the Blackjack suggestion. I'll give them a call.
I don't know the ins and outs legally on how it has to work, but mechanically if the pin is captive in the bore, in theory they could tig the hole shut and it would more or less be locked into place anyhow. I could definitely see a steel pin possibly interfering with the welding process of closing the aluminum over the top of it.
Again, someone who knows how it is done with an aluminum MD should comment, because I have no clue. I'd like to see the correct answer to learn for myself.