The chances of an "open convention" are slim. That's probably not going to happen, but what is LIKELY to happen are somewhere between 300-700 delegates and alternates who are anti-Romney. They could be Paul, Santorum grass-roots, and Gingrich grass-roots. While the convention isn't "open", it certainly will not be unified, and there would be too many delegates for the RNC to put a muzzle on all dissent like in 2008.
If the RNC were to do something silly, a walkout of that many people is noticeable. Romney saying something against the "libertarian wing", boos could be heard.
The RNC is going to have to tread very lightly here. I would not at all be shocked if Paul is given a hour on Monday evening.
That's not winning, obviously, but being in a position to scare the RNC is a lot better than it was in 2008, and they're going to have to give on some platform planks and possibly on RULES, which can only help in 2016 and 2020.