URL for querying metadata

So I'm thinking again on how to list all the fonts available from a given document, considering the stylesheet as the best place to define font directories. This may not be such a futile exercise as it draws the question about document metadata and the URI syntax to query it. By now the best I found is:
/~fonts/my.pdf
I like it because:

• URI parser can detect that "~fonts" makes no sense unless the MIME type is PDF.
• It doesn't mess the URI parameters which are about the document itself (not its metadata).
• It's easy to extend with other functions like word count or whatever, with no risk to create incompatible options.

I took a different way for error pages: these URIs look like
/broken.pdf/error.html
But meanwhile I had to find a workaround for displaying directories: a pseudo -.html document. So we could get an unified way to display metadata through some kind of "service" (including errors): /~fonts/my.pdf /~error/broken.pdf
Or, with Safari: /~fonts/my.pdf/-.html /~error/broken.pdf/-.html