Thinking about alumimum vs steel.
One can buy stem kits.
https://www.cycle-frames.com/bicycle...ing/STEM-LUGS/
https://www.framebuilding.com/NEWPARTSPAGES/lugs.htm (Bottom of page)
Yet, I don't think I've ever seen a seatpost kit.
The steel seatposts have almost completely fallen out of favor 50 years ago.
I am seeing this ITM post that might be interesting to work with. One could add a reinforcement, and finish it up nicely, well, if it wasn't chromed.
I do think that a good aluminum post like the Nuovo Record post has enough meat on it that it could be drilled without much risk. The posts are routinely machined to give them a SR look.