If your USB flash drive is booting PCDOS, are you loading the basic or the extended redirector? If I recall correctly, you need the extended redirector in order to login. It might be easier if you can configure your NAS box to share resources without having a login requirement, so that you can just map to the resource.
It is not that difficult to configure a WinPE boot for a USB stick and this gives you a much more comprehensive level of network support, so give some thought to this also.