I'm not sure I understand your question, but as to why this doesn't work, If I understand the question, the answer would be because of the nature of apache. If the directory structure isn't in place for the file requested then the request will never make it to the dhandler. In your httpd conf you have a FileMatch directive, you mght want to add a location match to force apache to dish out to the dhandlers those requests unconditionally

You have:

<LocationMatch "\.html$">
[download]

So you make a request to /nonexistantdir/foo.html; Apache shouts, no /nonexistantdir/ and dishes you a 404, this is faster than yeilding control to mason.

However, replace with:

<Location /nonexistantdir>
[download]

And suddently mason gets everything indiscriminately, if it is in that dir.