Peter:
You may consider a design where you put the polyfuse between the power input and your device input. Put the crowbar device downwind of the polyfuse so that when the trigger circuit activates the thyristor the polyfuse blows because of the load put on it by the crowbar. It is quite easy to size the current rating of the thyristor so that it can blow the polyfuse even if the polyfuse trip point changes by two or three times its nominal rating.

This will still protect your circuit as long as your trigger circuit is fast and accurate to set the crowbar at the safe voltage level.

You may also want to consider placing a bipolar high energy tranzorb device directly across the crowbar circuit (in parallel) that can clamp very high fast transients that are faster than the trigger/crowbar circuit can respond to.

Michael Karas