I have found something but I'm not able to re-distribute the code, unfortunately.
It was custom-made for a friends website, and he's let me use it on mine, but I don't have permission to re-distribute the code. Sorry
EDIT:
Just so you know though, the external form I use makes a connection to the forum database, and uses SQL queries to insert the thread/post directly into the database. I don't know if that'll be much help to you, but that's basically how it's done.