[mxfh]

I think you could just hook up an old pen plotter to your whiteboard.

https://www.youtube.com/watch?v=6_pwzqPk6Gg

[EDIT] whatever it has been done before:

https://www.youtube.com/results?search_query=pen+plotter+whi...

But this of mechanic is certainly unique and adds a lot of clumsy robot charm.

Wonder how easily you could store human inputs with a kind of (reverse) Pantograph type device for reproduction.

http://en.wikipedia.org/wiki/Pantograph