I honestly doubt that the browser vendors are at fault here, as far as I know other engines which also use HTML5 as their back-end, already have systems for advanced memory management. I'd rather think that this is something related to C2 not being capable of doing it.
We never really got a set in stone answer for this one I think.
I believe that Ashley once said something about unloading audio (?) from memory, either way my hopes towards the implementation of a system like this for C2 are pretty low.
I just wanted to point out that there is nothing wrong with the current system, It's just that some advanced (or mobile) games tend to suffer from performance issues and I think that this might be a good workaround.
I sincerely hope that you are right and nothing prevents Scirra to implement this feature. As I said it was requested by many developers in numerous topics and it's also one of my most anticipated features. But I don't want to turn this topic into a "C3 feature request" topic.