I don't know if it'll work or not. I'm sure the Wintermute's implementation of stencil shadows is going to vary from 3DGS; however those three lines are the key to getting this feature to work. I know we can add an effect of our own and getting that effect correct is one of two possible answers to this request.

The 2nd answer is having it hardcoded into the engine and available as a flag inside WED. Here's a real world example complete with unattractive behind-the-scenes pictures of a level that needs a lot of work :



Pretend that background is the background of the game level and everywhere you see green there is hidden geometry that closely matches the shape of the visible image. In fact the green is the hidden hull and pretend like there's a textured version that can be seen over this. Also, the green parts on that building are the possible locations for the player's shadow to appear; thus the reason why the hidden geometry has to match. The complexity of the hidden hull above this area doesn't really matter.

In WED, I would like to have a new flag and for every green surface, I'd like to be able to check that flag ON. Those green surfaces will be invisible, will not cast any dynamic shadows (if within the sphere of influence of a dynamic light..remember this is ultimately going to be pre-rendered or hand drawn, so any shadows will already be present); however the stencil shadow of any shadow casting models will render onto those areas if the lighting is setup properly for the shadow to cast in the direction of the green section.

I don't think this requires anything shader related or VS1_1 or PS2_0 type stuff.