I clip a PBSF to the rear head fitting 'strap' on my Giro Pneumo ...

That's exactly what I did, and it worked great — just clipped on there, and was aimed well.

I did that for a number of years until recently, when I had to replace my helmet. New one doesn't have that kind of stap/adjuster back there, so I had to zip-tie to the backside.

Works, but often I have to fiddle it back into the right aim. That was never a problem with the Bell helmet I used to have.