Recovery complete.
Wrapped the slide in gorilla tape then clamped tight in padded vise.
Placed a steel block onto the sight's front and started tapping with a 12 oz hammer.
Moved it a bit after a few judicious taps. Then the sight came out intact.
The drill had barely touched the sight steel, so a small (hidden) gouge is the only damage, an easy fix.
Then drove the drill bit out from the opposite side.
Drilling into a hole that's slanted on the bottom is tricky.
I've done perhaps a dozen S&W revolver sight blades w/o incident.
So, off to a machinist gunsmith friend.
Thanks and a good night to all!