[IlliOnato]

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.

[eyelidlessness]

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.