Tiny Tweaks to Custom Display Timings

I’d really like to find a way to make minuscule changes to display timings without causing the whole operating system to blank and freak out for a second the way it normally does. Wouldn’t it be neat if I could add just one extra line of vertical blanking for a few frames? Then get rid of it? This would have a very slight effect on the frame rate, but I’m hoping it would be tiny enough that my monitor (or really: my LED video wall) won’t notice.

I wasn’t optimistic, and indeed after a few days of looking around the NVAPI I’m no closer to my goal. Even just telling the API to “try” the custom timing that it’s already set to – THIS causes the cursed screen blanking.

Any idea? Or is this truly just NOT POSSIBLE?