I have (or used to have, it's broken now after several years) a bluetooth numeric keypad with some additional media buttons like the one below. It was great for many tasks, and almost as small as a remote control. I think there are a number of bluetooth or even wired numpads that can be used for this purpose. I've been thinking on a little project of tearing apart a cheap usb keyboard and use its controller (they are usually a very small circuit board) to create some custom one-row keyboard (say: alt, ctrl, shift, enter... etc) that could be attached or clipped to some border of the surface/tablet. Maybe this is not too hard to do.