file-relative-name and remote files

[Lars Hansen <larsh@math.ku.dk>]

file-relative-name assumes that filename and directory is part of the 
same tree except when we are on a DOS/Windows system and the first two 
chars in the expanded file names (the drive) are not equal. This 
approach does not work with remote files. With remote files filename and 
directory may reside in different trees. Since the syntax of remote 
files just is a prefix to an ordinary file name, it should be a matter 
of comparing prefixes, just as in the DOS/Windows case. However, remote 
files are handled by the file name handler mechanism which does not work 
for file-relative-name, since file-relative-name is a lisp function.

What would be the best way to solve this problem?