How about a simple band-brake (or a disc-brake) to stop the prop " in
case".
It will also work as a safety brake when starting and idling. But make the
brake handle cross the throttle so you can't accelerate when the brake is
engaged.
I'm sure
something like this could be worked out, but it wouldn't be an issue for draggy
things like gyros. You have to have a glide ratio before you can worry
about losing it :-)
Seriously,
the pre-rotator on the gyro we were flying can be used as a brake, and in fact
we did that when we shut the engine off to stop the prop. I'm not sure
that would be the best idea in the air, just in case there was enough energy in
the prop to actually turn the hydraulic pre-rotator pump enough to engage the
bendix on the rotor head.
"Keep the gyro" :)
I like the
way you think :-)
Rusty