There's still the matter though of "How the hell do I BUILD those damn walls in the first place?" though.
Aha! So while I was in the shower, I thought up this:
Use your method to reach all the way to the rock layer, and then the level beneath that(to drain out the water that spills into the first encountered rock layer).
Then, regress back up by removing the bottom layer's pumps and replacing those with walls and repeat.
Assuming a willingness to deal with annoying construction cancellations, the area exposed by removing the pumps will be drained by the top layer's pumps and will so be able to be walled in, leaving a 2x2 staircase surrounded by walls.
Repeating this for each layer going up, the shaft should be walled off.
Of course, this needs testing, but this is a recap what I've decided on:
1) Follow your method to penetrate all the way to the bottom, effectively creating a sorta pump-stack shaft without walls.
2) Once at bottom, remove bottommost pumps and replace with walls.
3) Repeat until shaft is completely walled.
Thanks alot, Lemurson!