Is there a way to preview RAW photos on an Android tablet from a PC over a network?
There are Apps which allows you to connect to your PC and control it with your Android device. These are generally referred as Remote Desktop Applications.

For Android, there are many remote desktop apps, one that I have tried is called Splashtop. I know LogMeIn, which is quite famous among remote desktop softwares, also has Android/iOS version available.

However these Apps are usually not free.

While these Apps can do exactly what you want, it can also do much more than just viewing RAW photos, basically it enables you to use your Android device as your PC's LCD monitor, so you can run any programs, open any files etc. Therefore these Apps tend not to be cheap.