But I'm cynical about the possibility to get a new bytecode standard out, without e.g. MS sabotag... extending it. Half the value of asm.js should be that if there is a working system already in place, the motivations for such shenanigans lessens.
The way to handle Microsoft in a situation like this is to ignore them. Support the same IR on Firefox, Chrome and Safari and then Microsoft can play along or go home. At that point the worst they could do is implement an extended version that allows sites made for all browsers to work with IE but then encourage developers to make sites using Windows-specific features that won't work on other platforms. That's effectively what they tried with Silverlight and you see how far that got them.