First, thanks to Vivek for providing this excellent Pen F extension tube adapter idea. Using that as a basis, I came across a micro 4/3 flange designed for astrophotography by a company called Hutech:
http://www.sciencecenter.net/hutech/borg/parts/index.htm
The flange has a 49.8mm screw thread. The Pen F extension tubes have a 50mm screw thread. They don't mate exactly, but it's close enough.
Referring to the photo above, take part D, and the Hutech flange, screw them together, presto, instant Pen F to G1 adapter! No grinding required, and no shimming!
I tightened both parts together as close as they would mate (it's a very tight, secure fit) and it focuses a bit past infinity. Unscrewing a partial turn will dial in infinity, so it matches infinity on the lens. I suppose some sort of set screw or Loctite could be used to secure the two pieces exactly at infinity, but I think I'll just tighten them to the max, and live with it until a commercial adapter is available.
A commercial adapter will also allow the aperture and distance scale to be right side up on the lens.
One other thing: if you look at the photo, there is a black ring on part D. If this ring is removed, it allows the lens to stop down to preset aperture by pressing the preview button on the lens. Focus wide open, press preview to meter stopped down & before taking the photo. Quick and easy.