didnt think it was possible on a 500 with it having no HDD and no usb or card reader slots only way i could see it being possible was with a NAS drive.
Have you checked your firewall settings and the like? Can you mount an NFS share from you PC onto your Dreambox? Theoretically, it's possible to do, but I wouldn't expect it to work first time. I did eventually get multiboot working, but not with the windows NFS server.