New thoughts.
I've been given a cast iron column from a Hardinge-Handsvelt sinker EDM. It's 39" from base to the top of the casting, and 10.5" x 8.5" across the column. It does have a big cast in L-shape that is nicely ground cast iron on the top. The column wall thickness is about 1/2"-13mm, and the corners are increased to about double that thickness. It does have some internal ribs but it's mostly hollow.
https://imgur.com/yGcqM1C
It's the same size as the welded steel column. I'd have to do exactly the same machining to it to mount the rails, and I'd have to mount the rails or the blocks on huge risers to fit the ballscrew in between them. It would scrape in much faster than the welded steel, though.
The question is, Is it going to be more advantageous to use my welded steel column, with it's slightly less than double Young's modulus, or the cast iron pillar? (Steel at 200GPa, vs cast iron at 124GPa). My welded column weighs almost the same, and yet I still need to weld in the top cap and internal brace in the welded column.
Basically, all I can see the iron casting having over the steel weldment is 10 times better damping, and 3" more height.