You're right, thrust vent force is not a self-limiting, if it always occurs when system reaches design pressure it should be on the operating categoty.
It's not a temporary loading...

This might helpful,

http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=6771

Still not sure with putting thrust vent force into Sustain case. But expert's opinion would be needed, not mine.

good luck with your work,