The subversion package doesnt add those 2 lines to the /etc/services
file :

svn    3690/tcp        # Subversion
svn    3690/udp        # Subversion


Maybe it could do it ?
And here is a template for the svnserve service through xinetd.
/etc/xinetd.d/svnserve :

# default: off
# description: The Subversion server.
service svn
{
        disable = yes
        socket_type             = stream
        wait                    = no
        user                    = root
        server                  = /usr/bin/svnserve
        server_args             = --inetd
        log_on_success          += HOST USERID
        log_on_failure          += HOST
}

If you dont add port lines into etc/services, xinetd wont start unless
you add a 'port = 3690' line into this xinetd service file.

I dont suggest this file to be verbatim-included as
/etc/xinetd.d/svnserve without a 'root' directory (like
/var/subversion, or /srv/subversion.. whatever..) which may be the
location for subversion repositories. If such a root directory is
created in the package (%dir /path/to/subversion), then remember to
add '--root /path/to/subversion' on the server_args line.

And i dont think svnserve needs to be root, so a dedicated user might
be Good Thing(tm).

This xinetd file thingy could be included as part of the package
documentation.