Questions for you to answer:
The post content has anything else than just the shortcode in it or not?
Should this be resettable, meaning to force it generate new content again, or one time generation only "no turning back" way of functioning?
This is a plugin or in the theme directly, or maybe external to wp altogether?
It would be good, if possible, to have a look at the actual code to properly assess what would be involved in getting this to function as you want it.
Regards,
Dobri