Doubly troublesome in my case given one of the effects (2D lighting) had core gameplay implications, but fortunately it proved easy enough to track. When a build is produced, amongst other things it creates a new folder <build_name>, containing various files. Key amongst them; a build log.
Popping open mine highlighted this little error:
at (wrapper managed-to-native) UnityEngine.Material:Internal_CreateWithShader (UnityEngine.Material,UnityEngine.Shader)
at UnityEngine.Material..ctor (UnityEngine.Shader shader) [0x00000] in <filename unknown>:0
at SFXPass.get_mat () [0x00000] in <filename unknown>:0
at PathfinderDoomFX.Awake () [0x00000] in <filename unknown>:0
So. How to dodge around this little conundrum? Fairly simple really; stick your custom image effects shaders in a Resource folder. Ordinarily of course, assets are either loaded via the Resources.Load method or by being assigned in the Inspector. Unity can scan through the latter case and pick out any asset that's being used and thus needs including in the project, and it handles the former case by just automatically including everything in Resource folders blindly. Shader.Find() is one of those odd methods out that can reference things the above techniques would miss, and hence - whilst it will work in the Editor where the game has access to all the assets, even the non-final ones - fail in a full, leaner, standalone build.
Finally, there's also the 'Always Included Shaders' array in the Project Settings > Graphics rollout. This can be useful if, like me, you're re-using some of the Image Effects shaders but not the components themselves (you have to write your own scripts if you want them to run their calculations on a rendertexture rather than the screen) and don't want to touch or rearrange the Standard Assets themselves, which obviously rules out sticking them in a Resource folder. For your own custom Image Effect shaders though I would recommend the Resource folder option over the Always Included array; with Resource, they automatically get included simply by being there, making things much easier to maintain.