The greenpoison payloads are open-source, you'll probably just need to have the N900 go into hostmode, send commands and images, then send more commands. The problem, however, is the timing. If you cant send a certain command (too lazy to look up all of this on the iDevice wikee) fast enough, you get stuck on the white screen (pwned dfu).

And yeah, about the IPSWs (redsn0w approach), if you want to JB resonably quick, you have to prepare them from a computer.