I found a workaround to the refresh bug: in another terminal window, I run:
watch -n 0.3 xrefresh -geometry 1x1+500+500
That uses the xrefresh program to create a 1x1 window at 500,500 on the screen every 0.3 seconds. The refresh is enough to make sketchup update itself without totally disrupting the screen.
Neat! The main difference seems to be that drill.rb only drills round holes, and it drills them with only one click.
Not being able to drill holes in components and groups (even if the component is in 'edit' mode) is pretty annoying, though. I had hoped that the API wouldn't make me worry about this distinction. Surely there's a standard way in ruby to draw on a face regardless of whether it's part of a component or group?
I'm making a cabinet in sketchup for export to a shopbot, and it's really a pain to make all the drill holes in the pieces. So here's my first ruby extension, drill.rb. Select it, type a diameter in the VCB, then click on a face. It creates a hole of that diameter (and leaves that "drill bit" installed until you change it) in the face then pushes it through if it can find an opposite, parallel face.