For anyone who uses the control room talk back functionality, the free Sound Radix Mutomatic can be placed in the insert of the talkback channels of the control room mixer, so when you play or record it automatically shuts off the talkback, you can set it to happen only for recording, or both playback and recording. the plug in manager reports it as 0 samples latency and seems to work very well for me, I have 3 instances running at once.
Of course Nuendo already has Auto disable in the CR preferences, but it global to all talkback. using the plugin you can set it on a per channel basis in more complex setups such as multi room band recordings.