On Thursday, March 15, 2012 8:54:15 AM UTC-7, Tim Wescott wrote:
How would one make nice sharp-cornered, clean-sided (I'm not sure of the
surface finish, but mirror-bright would be nice) square holes, about 0.1"
on the sides, off center from the axis of the bar (so not square to the
bar surface), in a round steel bar about 0.75" in diameter?

Why not mill a square slot in the end of the bar, then attach a cap?
It's certainly easy to do, and the bar is big enough to take a variety
of cap-fastening schemes (weld, rivet, bolt, pin, threaded section).