It is kind of a hack, but you could toggle the visibility of a plane that covers the camera. Give the plane a holdout material and the frame will end up either black or transparent if you have transparent activated in film settings. That way, the frame will basically be rendered instantly, wasting very little render time.
And yes, I am assuming that you are using Cycles
Too bad it’s not you.
I am pretty sure I saw an addon or script to add frames directly in the Timeline. Was working on windows only so I didn’t keep it because I am on Mac. It may not do the NLA job though, I don’t know.
Thank you for the script.
So as far as I understand it does the same thing as grabbing in Dopeshhet or F-curves.
I guess many people are facing that problem and I am surprised that no one had made a one-step insertion addon.
Hope this will come in the coming Blender versions.