Single Projector 3D Mapping with 2D Overlay

[Daniel B Chapman]

I'm stumped on how to organize this project and I'm looking for some ideas.

Here's the premise: I have some scenery that, ideally, I would like to use 3D mapping for (walls that move) but we will still have a fair amount of traditional 2D content that will be used in other sections of the stage. I only have a single projector. What I would like to do is use a traditional 3D model with each surface mapped as a virtual display that I can then animate off sensors or manually but I want to maintain access to a 2D timeline that comes out over that content.

Is there a way to take the output from the 3D Projector and use that as 2D content in the background of my 3D model? 

Here's a quick sketch:

Basically I need this all to "pipe" through one projector and I would essentially layer over the 3D modeling (but I do want precise textures on the surface of this scenery. 

I'm trying to avoid this scenario where I have a mesh that needs to be aligned orthogonally to the 3D Mapping projector that, presumably, will need a lot of calculations to align.

I'm hoping someone has a clever way to lay this out. I would really prefer this be programmed entirely in Watchout given my staffing requirements for the project. I would also like to avoid creating an extra monitor, scraping that into NDI and piping it back into Watchout as a 2D element as it would be incredibly difficult for previsualization and rendering.

Thanks! Any help or ideas are appreciated. I really don't want to drop back into something like Unity/Unreal/Touch Designer on this for the mapping portion.