Does a hack that utilizes register.php exist in the form of...

1. user fills out a customized register.php form
2. the form info is posted to a thread on the forum

I can't find anything like this, one person said something about an FXP Pack but it turned out to be full of lame FXP mods and what not.
Any help would be appreciated.

There were several mods hacks that did these things. This is known as "vouch system". Sadly for you, this community removed all fxp hacks that were once here and is no longer supporting these.
Last i recall there were other several "fxp hacks" communiies, that had these "fxp mods" but i dont recall their domains.
Google for "fxppack" (was written by ShavedApe) - that will lead you to some small community that might have the answers you need (be sure to read & follow rules if you dont want auto ban).