Don't think it's possible with this router.
I tried the 'Firebug' trick as described in this thread :
http://forums.dlink.com/index.php?topic=5413.0
Router's status correctly shows the static route to LAN but it still doesn't function properly when accessed from the other subnet (weird problems ... loses connection to my NAS after a while, can't ping my outdoor Access Point, etc.).
Switched to another leading brand of router that supports this feature and everything worked as intended.
Disappointed that I had to ditch this router (with firmware 1.04) as it was really stable (& fast !) for me. IPv6 works great too. Could have lived without the PPTP VPN Server feature but not without Static Route to LAN.
