I don't know how you access your drive from your laptop. Do you have to run an application to gain access to your external drive on your router? Does it allow Explorer to access your data through a UNC name, an IP address or a drive letter? Unless there is "an App for that", you will more than likely not be able to connect directly to it with your iPad. But if you can map or connect a drive letter on your laptop to your external drive, then you could reference that drive letter, at least with Air Video, as the source for you data in the server companion application running on your laptop that is used to stream to your iPad. Maybe Zomocast could do the same.
BTW, the new Western Digital My Book Live (2TB) NAS allows access (local or from the Internet) for iOS devices through an App linked to a MioNet service (just saw that info on cnet.com). As far as iOS devices are concerned, it might just be for Photos (but full access for PCs). Look it up carefuly before buying...