No there's not a whole lot of code that will run in less than 1 scanline, BUT there are a few very good examples that will, and many more things run under WinUAE with JIT and immediate blitter or on a faster system might as well.
Try something like a simple fade to black of a 16 color screen in 12-bit RGB on an 060 and you'll be looking at 1 scanline or less.
I'm just saying the check for the MSB switch from 1 to 0 is safe, short, and works for all programmed displays. If people wrote solid code to begin with they wouldn't have to sit in sorry hindsight when their code freaks out on faster systems, and people like you wouldn't have to sit and WHD-fix poor games that won't run on A1200
