I believe I found a bug and will continue to believe so until someone can point out a user-error on my part.
I work with MPC and the Mill often and they use Arnold 184.108.40.206 (v2.2.1) in C4D r18. Yes it is not the most up to date but I checked the release notes to make sure this wasn't fixed in the newest version of Arnold C4DtoA.
The problem has to do with custom LPE's. If I'm writing a custom AOV, c4dtoA simply does not recognize the LPE or render it.
I've tested the SAME LPE's in Maya and it works perfectly. Why does c4dtoA does not support custom LPE's? Yes the UI is all there and is seems that it should be working but simply put, the same LPE's the work in Maya, do NOT work in C4DtoA.
I know it's not different as Arnold LPE's are the same regardless of what 3D platform you're working on.
If you happen to have Maya + C4D, try out this LPE:
Make sure to place an object within a tranmissive container, and you'll see exactly the issue.
Maya's renderview will indeed show the custom AOV, C4DtoA will not. Just in case this might be an issue affecting IPR, I've tried rendering but c4dtoA definitely needs to be updated so LPE's can work in Arnold.
Let me know if I am wrong about this, I'd much rather prefer to be wrong than wait on Arnold to update c4dtoA so it can support custom LPE AOV's for more intense projects.