This is just the absolute basics, but this AppleScript will repeatedly hit the Next Track button for you, and then add the current track to a pre-made playlist. You'll have to have the genius mix already playing, and a playlist named Genius_Mix already created before running the script.
I'll probably get around to making it nicer, but if you're itching for a playlist now, it should do the job.Mac OS X Hints
http://hints.macworld.com/article.php?story=20090918161939914