Hidden random post # revealed after x days.

[leftie]

On one of my forums we have a small prize competiton where members who reaches the said post number gets the prize, ie; first post will read, " blah, blah, blah, post # 436 will receive the prize. Everyone knows what to aim for and it spoils the fun.
My request is that in the first post (by admin) they can state the prize but there is a little blanked out part that states the post number, ideally this will be randomly selected within a range that is selectable within the acp settings. Once a time has expired, again selectable, the blank box reveals the post number.

Thanks.