How can I draw vectors onto a Texture2D?

I need to achieve an effect like this:

That is, to be able to overlay a dynamic texture onto a dynamic mesh, and have it follow the mesh's contours. I believe I can achieve this with a projector, or by using Decal System Pro. The main problem I'm having is, how do I dynamically draw the shapes I need onto a texture in the first place? There doesn't seem to be any built-in library for this, and I'm having no luck hunting down sample code that uses System.Drawing to draw onto a Unity Texture2D.

Anyone have any ideas? Cheers!