Hi Mike,
It looks like the streamed videos from twitch have disappeared (as they do after a couple weeks). Official support for the "custom buffers" functionality will be a part of the 3.10 release, and I'm working on putting together a gr-cuda OOT that will capture the basics of how to use these custom buffers and do processing in CUDA.
In the meantime, the OOT used for the hackfest session is here:
Mainly, take a look at the differences between the main and custom_buffers branches to see how custom buffers are used.
Thanks,
Josh