Still not comfy with what bolt would be a best option. And on eBay, a used Velox RD with the nice bolt (red "C" center) starts at $40, a lot for just a bolt.
Originally Posted by
oneclick
Chain-tensioner for a freewheel machine with double front rings.
That sounds so 1930's in terms of aesthetics!
I like it; presumably a tensioner comes with a mounting bolt.
Originally Posted by
merziac
The empty hole bugs me too, I always thought there should be like a chainring bolt, maybe with a tiny reflector attached.
So that got me looking at chainring and other crank bolts online -- alas, none seem to be M10. But it was worth checking!
Originally Posted by
DiabloScott
Not sure what else it would be good for.
I could imagine a clever engineer could devise a backward-swinging kickstand to mount in that hole. On urban bikes -- curb feeler?
Originally Posted by
Fahrenheit531
Instead of repurposing the hanger, just get yourself one of these oh-so-sexy White Industries freewheels to divert your attention...
Ooooooohhhh... shiny! 
Shiny is always a winner. My current project is IGH so I don't need a freewheel, the sprocket is shiny silver but not exceptionally so. I am just not sure I want to drag it off to the chrome $hop.