Note that OptiX 7 applications mentioned above are published in the meantime.
Find the link here: https://forums.developer.nvidia.com/t/optix-advanced-samples-on-github/48410/4
Those are using direct callables extensively.
The --relocatable-device-code=true (-rdc) option is required since CUDA 8.0 because OptiX callable programs are functions which don’t appear as calls inside the PTX module and the CUDA compiler will optimize them away as dead code otherwise.