[rewrite] how to forward env. variable via URL rewrite ?

Hello,

I'm facing various problems on MacOS X Server that have lead me to setup
a 2-apache web server (in order to use mod_dav_svn with mod_auth_apple,
each, unfortunately, running on a different apache version).

I have this setup :

front apache : 1.3, - mod_auth_apple, providing authentification
against the university LDAP/OpenDirectory
- mod_proxy, proxying requests to "back apache"

back apache : 2.x, - mod_dav_svn; providing SVN over HTTP

Everything works fine, except for one thing : mod_dav_svn is not aware
of the name of the user who logs in on "front apache". Then, all the
meta data of Subversion are not fully set.

What I want to do is to forward this information to the "back apache"
using rewrite rules.
On front side I have :

RewriteEngine On
RewriteRule ^(.*) $1/u=%{REMOTE_USER}/ [L]

On back side I have :

RewriteEngine On
RewriteRule ^(.*)/u=([^/]+)/(.*) $1/$3 [L,E=REMOTE_USER:$2,P]

I tryied with E=REMOTE_IDENT:$2 too, and with both, with no luck.
mod_dav_svn still can't find the user name...

Any idea ?

patpro