Yes you can set the Talkback Dim Level in the Main section of the control room. This setting is not displayed by default. You need to click or control click in the control room main section bar to display it.
The talkback dim can be applied to cue channel as well (file menu, preferences, vst control room).
Talkback level can be set independently for each cue channel in the control room.
You should use a midi controller button for talkback control. I did use a DJ controller for that but many other midi controllers are usable for this simple task.
At the same time you could use this controller to get most control room controls (levels, monitor selectors, dim, ref level, click on/off…) at your finger tip.
If you don’t want an external midi controller the talkback function can be assigned a keyboard shortcut like almost all Nuendo functions : File menu, key commands, Control Room, Talkback on/off.
The talkback manual control is very nice : can be latched if you just click the button, or can be momentary if you keep your finger or mouse on it.
Last, you could use a free plugin for talkback automatic control, for example MuteOmatic.
Last, there are options in Nuendo to automatically deactivate talkback when play or record commands are issued : file menu, preferences, vst control room.
Unfortunately we still need a plugin to activate the talkback automatically. I did ask for that and for a listen mic control in the control room. It’s a simple and needed addition for a high end virtual studio software. Perhaps in version 10.2 …