Exceeding Vmax rating of PTC Fuse
PTC has its place, but over voltage crowbar is seldom it, applications like USB overcurrent protection are more the sorts of place for those things (and even then there are usually better ways). 

One trap with PTC is that it derates HARD with elevated temperature, for a design current of 600mA, I would be using a PTC not smaller then a few amps if I was using one at all, your chosen part will be on the edge of tripping at 600mA if the in case temperature gets up to 60C (Which it well might). 

The other trap is in the pathetic interrupt rating that these things have, compare with a real fuse from someone like schurter.

I would pick a standard fuse of about a two amp rating, not too fast blow and size the thyristor to suit, job done.

As to the thyristor dropping out and the thing recycling, that is on your design chops, could happen, not guaranteed to, and possibly your DC/DC converters will soak up enough current to hold the thing tripped (Check this at minimum temperature as well as maximum at both extremes of supply voltage).

I don't like the things because you have to spend far too long dicking about with an environmental chamber to be confident that they will work perfectly in an application like this.