I still think most of you are looking at this wrong. The most important thing for Johnson is name ID. Voters have to 1) become aware of his existence, and 2) know who he is well enough to realize that he's not a completely insane crank candidate like so many 3rd party candidates are.
To that end, the most important thing for him is for the media to cover him more, and actually treat him like a 3rd option in the same way that Ralph Nader was covered in 2000. Even while people understood that Nader wasn't going to win, he was treated as a legit protest option for people who didn't like the major party candidates. Saturday Night Live had jokes about him, in which they didn't have to stop to explain who he was or that he was a 3rd party candidate for president. He was getting enough media attention that the average joe knew that stuff.
Can Johnson achieve that level of recognition? Well, maybe not, but he sure would be more likely to get media coverage if people as prominent as Mitt Romney endorsed him. It's not just Romney fans who would be impacted by such an endorsement. Anyone who sees the additional media coverage generated by endorsements like this would see increased coverage of Johnson.
Even the stuff about Romney hooking Johnson up with rich donors I think misses the point. Johnson isn't going to be able to raise enough money to advertise enough that he can boost his popular vote %age in any meaningful way. Running a nationwide campaign is just too expensive. Free media is going to be far more valuable for him.
Bill Weld (Johnson's VP, a two term governor of Massachusetts) raised 5 million for them in the past week.
$5 million is a miniscule amount of money when we're talking about a nationwide campaign. How much is Johnson likely to raise in total?
