Membrane deformation mask

Creating a mask to separate intra- and extracellular portions (a la Marc):

This is a problem I've been thinking about for a while, but only came up with very convoluted answers. Here is Marc's simple and effective solution:
1. Create a cube and position it so that it intersects with the undulating membrane
2. Select the cube, select the membrane > Create a wrap deformer
The cube should deform the same way as the membrane.
3. Render out the cube completely white and the membrane black - bring the mask into AE for tinting, yay!

This way I can also use the mask to tint the objects without having to mess with the light colours!