Don't think there are pathtracers out there with render passes support
I think you're right - a full channels/passes system almost defeats the object of using pathtracing. I don't expect the passes/channel feature to be available for it. Direct lighting, yes - pathtracing not so much.
Implementing Alpha mattes would be nice though, where you can just render out a white on black pass for individual materials in the scene. This would give compositors some better control for isolation masks and colour correction.