For the most part, third party voters are principle voters, right or wrong. No chance of Mitt fitting in there.
There are 49 "third parties" currently, I would describe most of their members as something different than "principled" but I know what you meant.
On the other hand Romney has a big pool of Mormons that if they weren't ordered not to vote vote for him, may give him 48,00 votes which would beat Alan Keyes's 2008 effort.