you should be alright without the heat treatment of the welds. Just leave the frame alone for 2 weeks and it should settle down and strengthen. The main reason the majority of frames are heattreated is to speed up this process. just make sure they use backing gas, as this will take the majority of the contaminates out of the weld as it forms.
the main concern i would have is the heat the aluminium produces when its welded is likely to warp the seattube, especially since it will be very thin walled. make sure you get someone who knows what there doing not a man in a shed as it will become alot more costly in the long run.
it might be worth talking to a specific frame builder about this as they will be set up for a job like this. theres plenty of them about now-a-days.
The company are a marine engineering outfit and have a very good reputation. I take it you mean leave it two weeks before I get it powder-coated so as not to lock-in any gasses?
I know you’ll laugh OP (and others) but this seems to me to be a perfect job for carbon fibre & epoxy wrap!
That is actually not a bad idea at all and theoretically, I could use thinners to remove it should I ever change my mind.
Hmmm….