You're right, this is a limitation compared to forward rendering, where shaders are applied to each object individually (you just need to skip the lighting calculation to get something unlit). Since Crystal uses deferred rendering, everything naturally receives lighting, so to get something "unlit", you need to light it. The other alternative is to use the Combine pass, but it can only be drawn after everything.
Viewing post in Crystal - 2D Lighting Engine comments
OK, I get how the engine works now!
The "unlit" issue (excluding objects from lighting) is something I think I can work around in my project now, so that's solved for me.
However, the issue with the "EMISSIVE pass visual change" is definitely getting into the engine's internal settings, so I'd like to send you a few more detailed questions via email to discuss it further!
Thanks a bunch for your help! :)