if I'm not mistaken, this eco-tour agency did organize trips into the Forbidden Zones in Laos. I remember reading about it on their website. Inthy Deaunsavanh, the owner, is real cool guy and he also knows all the regular bikers based in Vientiane. You may get useful info on getting to the FZ.
www.greendiscoverylaos.com
www.greendiscoverylaos.com