I'm afraid we can't do that, Dave.

/HAL
The reason we only make the "edit post" button available for thirty minutes after you post is an anti-trolling measure. On the old forums, we had a few instances of people deleting posts in a huff, deleting posts to make a discussion appear one-sided, deleting posts to obfuscate the fact that they'd been downright nasty so the mods didn't find out and so on. We decided that removing the ability for people to go back and do that outweighed the downside.
Thirty minutes is usually sufficient for people to notice and correct any spelling errors or similar they've made; and if you've had an epiphany that makes you think something you said earlier is incorrect, we encourage you to start a new thread about it!