Well I guess if you really want to draw in many customers to help with funding the beta - Perhaps offer a limited edition *Something* with their purchase... such as an unique insignia next to their name to those who supported you before the final game came out... after the game has been released no one is able to receive this insignia any longer... Or something along those lines
I see that there is already something to this effect in place but im not sure if everyone will get them once the game is no longer in the beta stage...
They could also open pre-orders for people without beta keys already, hence giving them access to beta. I know if I didn't have a key I would pre-order it for one.