Change extended attribute on each save of a DCR

System

I want to set an extended attribute on a DCR when a user saves it. (The value depends on one of the fields inside the DCR) All idea's on how to accomplish this are welcome....

At the moment I'm trying to call a CGI script with the callServer method of the FormAPI. This works, but I can only send the filename to the CGI and not the path as there is no method to request that information in the FormAPI. So inside the cgi I know everything I need to know except the location of the file!