Now that's a very difficult question. Right now we don't have a decent approach for this. However, there are a couple of options of faking shadows:
- Rendering into a separate texture and using color-manipulation tools like [ChannelMixer] and [ColorGrade] in combination with [BlendMode = Multiply] might work as a fake.
- Sometimes, placing shadow-blobs on transparent planes below the geometry might work.
- Applying SSOA might help faking
- Backing fix lighting in a software like Maya or 3dMax could also work.
That said, since all required information (z-buffer, camera-matrix, light-positions, etc) are easily accessible, implementing an Operator for Stencil Shadows or Shadow-Maps should be straight forward for a capable coder. If somebody wants to give it a try, feel free to ask more questions.