James Watriss
I have (no particular order, and not to all planes)
-Lapped the soles flat on a long flat surface
-Ground sole and sides flat and square with a (dull) disc grinder.
-Used said disc grinder to flatten the sole of an old, cast iron infill.
-Drilled and tapped the sides of cast iron planes to install set screws near the mouth to make lateral adjustments more controllable. (Even did this to my L-N #4.)
-(No comment on doing this to the infill plane.)
-(No comment on installing a second set of screws towards the top of said infill, to fix lateral play, and make set-up easier.)
-Followed David Charlesworth's advice (found in the first book) about using carborundum powder to grind mating surfaces to match each other after scraping them down, filing the forward part of the mouth to clear chips, and so on.
-Smoothed out and refinished totes.
-Ground and sharpened blades.
-Swapped knobs and totes with other planes that I found to be more comfortable.
-Used black nail polish to touch up chipped japanning.
-Very seriously considered painting a buddy's Bedrock #8 with a lovely shade of pink after he stole the bolts out of my bench.

All of the above is fair game... It's all about making a more useable tool.
But if I find out you drilled a hang hole anywhere on the plane, you're out of the club.