I'm no expert, but I think there are a few flaws in the thinking.
I don't thin any amount of post extension in the frame will help.
First it'll be butted (i think these where) so it's no mans land down there or it will not have been reamed any way.
It us probably over tensioning of the binder that has pulled the Alu a bit and happens to be a weak ask spot in the welding position.
Spreading would happen when you open and close the binder as it's bending that bit a little.
This is just an idea and I wouldn't do it without more input from more experienced members.
Because the crack isn't that advanced down the tube, couldn't you drill a small hole below the crack to stop it spreading? And run a longer seatpost way below the minimum insertion mark to distribute the load more evenly along the seattube.