It does work, but contrary to iZotope’s suggestion you need to insert ‘RX Monitor’ plug-in on Audio Track instead and activate Monitoring (the little speaker icon). If you don’t see it, you need to add it in your ‘Track Control Settings…’
It appears that iZotope haven’t tested their own software with Nuendo/Cubase.
I just put it on a group track. Then solo-defeat that track and choose “RX monitor” as an audio output in RX standalone app.
I did have a glitch at one point or another though and since I’ve switched to a better interface I can actually access the interface directly using *both *Nuendo and RX, simultaneously. So now I just don’t use “monitor” at all, even though it’s still in my templates.