[Vitaris]

I found it! Sharing with people in case they're interested in how I got this to work. I can't find the original source (I know it came from somewhere), so if anybody knows, let me know.

Login to your Admin and go to Plugins and Products > Add New Plugin

Set the following fields:

Product: Vbulletin
Hook Location: showthread_getinfo
Title: Whatever you'd like to call this plugin (eg. Welcome Thread Inverted)
Execution Order: 5 (This is the default setting I have, I guess there's no need to change it)

Plugin PHP Code:

if($thread['threadid'] == XXX) $postorder = 'DESC';

Plugin is Active: Yes

Where XXX is the thread ID, and DESC is used to show threads from newest to oldest. Svae this plugin and reload your thread. It should now be inverted.