As long as the d-pad is the only "button" on screen it should be possible. In technical terms, it would just be a matter of taking the vector difference of the point-of-touch and the centre of the intended d-pad position. Then you'd map the vector to one of the eight possible directions. Or you could just clamp the length of it and you'd have an analogue "stick". Kobo Deluxe essentially does this for touch control already.