I guess support for XML would be tricky, because XML is just way more complex format than the ones already supported. It is still essentially a tree, but with additional structure.
Representing elements and their contents is easy enough. But attributes, comments, processing instructions, entities... And remember, an XML document can include a DTD (it does not have to be in a separate file).
To present it as a file system in a useful, non-convoluted way? I will be very, very interested if it's possible, but not holding my breath.
On the one hand, I can’t help but point out you forgot to mention the other big inherent complexity that would make XML-as-FS a uniquely complex beast: namespaces.
On the other hand, I can’t help but point out that a related technology comes very close to demonstrating how you might map XML to a file system: XPath. Probably the biggest issue would be syntax, and again largely due to namespaces.
Representing elements and their contents is easy enough. But attributes, comments, processing instructions, entities... And remember, an XML document can include a DTD (it does not have to be in a separate file).
To present it as a file system in a useful, non-convoluted way? I will be very, very interested if it's possible, but not holding my breath.