Not an 'auto release', but some info from this document :

http://www.glidingmagazine.com/pdfs/...20Rollings.pdf

"That of course is the reason that attempts to produce a hook that released
if an certain angle was
exceeded were unsuccessful. The quite small angle needed to trigger
the“Kiting”when the
glider is pitched significantly nose-up is not much greater than the amount
of out of position
commonly experienced in turbulent conditions. We did build an experimental
hook and tried it,
but, set to an angle that prevented“Kiting”it occasionally dumped an
innocent glider in
turbulence, and set to an angle that prevented that, it didn’t prevent
the“Kiting”. What was
needed was a hook that responded to the vertical component of the load, not
the angle at which it
was applied, and that problem we decided was beyond us (at least in a form
robust and foolproof
enough to be attached to the rear end of a tow-plane)"

Solved by

"We solved that problem on our tow-planes by replacing the bolt that the
hook latches onto with a
small roller bearing. So far as I know, no one has tested the Schweitzer
hook as fitted to a glider,
but I would not be surprised if it exhibited the same characteristics at
high loads."
http://www.facebook.com/KranskopGliding