Thanks! Confirming that cv::Mat::setDefaultAllocator() not working for me was an error on my part. Resolved now.
One clarification: pinning with cv::cuda::HostMem is useful only when new cv::Mat objects are allocated, correct? And you have to know the size and type of the allocation also? For example, if I want a pinned cv::Mat after a call to cv::imdecode(), I cannot use cv::cuda::HostMem, unless I know in advance the size and type of the image being decoded?