The easiest way, you need:
1) Video Camera
3) Switcher/Mixer with a luma/chroma key.
Simply feed the camera signal and signal from tablet into the mixer, set it up so that the writing is keyed over the video. Voila. Of course, mixers are expensive, but renting one might be affordable for you. Since it's live, make sure to choose one with a 2 frame delay at most.