I'd only use a post like that if it extended into the seat tube well past where the top tube and chainstay join the seat tube.

I've busted rails on a saddle just from landing on the saddle when I missed a pedal clip in - and that didn't have the lever arm that monster setback is going to have.

If it's a hardtail MTB, even hitting a bump hard could easily be enough to bend the seat tube with a lever arm like that if the post extends into just the very top of the seat tube.