It's a pain in the arse
If your recording a game then the best thing to to do is record it in sections.. I play freeze tag so it's not an issue to hit the arrow keys as I'm usually frozen most the game. I don't know how many times I've hit a key by mistake on a long demo.
If you have a long demo I will use /timescale 1 to 4 or whatever is appropriate which will play it in faster motion. If you play it too fast and skip by it then you can always look at the time and note where you want to start/stop recording.
I just practice on Kick52's demo
ps. I dont think you can record playback of a demo