[plt-scheme] servlet question

Is there a straightforward way to determine the absolute path of the 
'default-web-root' from a request uri to a servlet? I would like to have 
a servlet-specific configuration file that lives in the same directory 
as the servlet; I have implemented a hack that makes some 
not-very-portable assumptions about the web server configuration but I'd 
really like something like (get-real-path uri). What am I missing?