Instead of scattergun guessing, why don't posters here sample some balanced websites, get a feel for the right wing posters, and see what the opinion is of Rudy. Big hint: you'll be shocked, and throw away the cliche "Stick a Fork in Rudy" line.
The posters on progressive sites who are writing that are the ones who were trying to pretend that Mike Huckabee would magically emerge solely because he fit their preferred blueprint of a contemporary Republican.
The nightmare scenario is the GOP handicaps electability properly and identifies Rudy, while we ignore it based on '04 and come up with Hillary. That possibility scares me every day. Newsflash: both are favored right now and favorites hit at very high percentage in the presidential nominating process.
It's ridiculous to assert the GOP will follow a Rudy nod with a third party run. The entire point of a Rudy nomination would be a concession that an atypical nominee is needed in an unfavorable climate like '08. You don't lance that with third party candidate who steals from your own guy. Make no mistake, both parties will work feverishly for their standard bearer, regardless if he/she was the first choice of the so-called base of the party.
I have a wager on Rudy but I'm rooting for him to fail. He'll definitely steal from our soft supporters in a general election. That's another reality via the balanced sites. Posters, particularly males, who are moderate and even lean our way are more excited about a Rudy run than any of our candidates.
I think it's ridiculous to say that a hypothetical conservative third-party candidate would steal a very large portion of Rudy's potential votes if he is the nominee, but that's not what could make the difference. If this person could potentially take five percent of the votes that Rudy would otherwise get, it might help us in, say, Michigan or Pennsylvania, where the race could be close.
The question really comes down to what you hint at: how big of an exception is the Republican base really willing to make. Right now, it's not entirely clear.