SSR vs High Voltage Transistor.

Yes you could use this similiar to useing a mosfet with lower voltages, he only difference would be you need to be more careful about spacing
In a current project of mine im actually using 5x nchannel mosfets rated to 600v and I have plans to actually switch up to 500v with that,
You could probably make your own ssr with the right mosfets, that's probably what's inside most ssr's anyway