Apache rewrite is what I was thinking *would* work for masking the webdirect URL. However, I've never had the time or need to fully explore it to try and get it to work.
Since the /fmi/webd is hardcoded in some form in FMS (not sure how or where, it may be compiled into FMS itself), I'm not sure exactly how apache rewrites would affect that. That's the only risk I see in attempting it. You will notice that there is no fmi or webd folders on the server, the path is dynamic from somewhere.
I CAN tell you that two other common methods of URL masking do NOT work:
1) embedding WebDirect as iFrame content.
2) PHP's file_get_contents() function
As for the apache rewrite itself, your code looks sound.
FWIW, I have modified the httpd.conf file before so that PHP uploads go into the documents folder so that I can pull PHP uploads into filemaker later.
Since you're in uncharted territory, I'd see what you can do to get this to work. If you break a new technique, please let us know!