Is there a way to trick the IIS Manager into accepting a directory path as part of the Default Content Page setup?

Do you mean to have IIS show a directory contents instead of a content page.  Hence, not use default.htm or default.aspx but rather show all the files listed in a directory?  If that is the case you can select "Directory Browsing" and it will display all files in a directory if there is not a default document present.