What'll happen is this...
The rich euro countries must realise that it's a choice between them hanging on to their money for now but having a financial disaster resulting in no euro, no banks and probably eventually no money. Or, they can bail out the debtors in the eurozone, resulting in a continuation of the status quo, albeit with less money for themselves and an urgent need to reform how the euro works.
The decision really rests on how much Germany and France want to keep the euro (i.e. how much they're willing to pay to keep it). When push comes to shove, if they decide it has to be rescued (and I think they will), the fact that there's no legal way won't matter. These are politicians - they make the laws (or they re-interpret them as necessary).
IMHO, Germany and France shouldn't be that bothered about having to bail out the PIIGS. The PIIGS have suffered from having an over-valued currency (the euro), while France and Germany have profited from having an under-valued currency (also the euro). If they want to continue profiting, it's only fair some of that gain goes back to the PIIGS who are keeping the exchange rate down for them.
So a transfer union of some sort, but the reason that hasn''t already happened is because the rich euro countries don't want to sign a blank cheque. While they can, they're determined to make the PIIGS reform their economies as that will make the eventual bill as small as possible. But I think they know the bill will eventually fall on them, probably through some form of QE. That'd be the fairest way, as all sectors of the euro economy would then pay via the effects of inflation.
I know the eurozone politicians are acting like they're stupid, but I think most of them know how this must play out. They just don't want to commit before they've squeezed as much as possible out of the PIIGS. Similarly, China, Russia, Brazil, US, UK, et al. aren't going to commit to helping the eurozone via the EFSF until they've squeezed as much as they can out of Germany and co.
It's all politics of course, but in this situation it's also good economics. Those who have made mistakes must pay, but only as much as they can afford to pay without wrecking the whole financial system. That's what's being negotiated at the moment via a process of brinkmanship.