No, the HSync must always happen exactly every 64us and have the same width.
If you don't have it every 64us, the screen will distort as the monitor will slowly catch up with the sudden and unexpected change. Example of demo doing this: Plasma Pas Cher (going from 63 to 65 us between hsync which is reasonable). There are old demos from the 80s doing much weirder things.
If you shorten the HSync length, the screen will be slightly shifted to the left (not exactly a whole character, you can make it move by about a pixel). Axelay used this in his games to implement smooth scrolling. If you shorten it too much, at some point the blanking will not coverthe PAL color burst anymore. The CTM doesn't care about this, but if you use the CPC with another display (TV, video projector, Commodore 1084s, ...), the display will take the color of whatever is in the color burst area (usually border color). The result is usually a smooth gradient from border color to black over a few lines. There was a picture of a test demonstrating this during one of the ReSeT parties, but I can't locate it at the moment.
Most of Batman Forever distorsion effects are essentially line-to-line screen splitting (CRTC screens 1 line high, changing R12 and R13 value at each line) and clever palette changes.
The counter for the HSync length is probably only 4 bits wide, as is the R3 setting for HSync. So, you can't set a long width, it will count for at most 16 chars and that's the maximum HSync length.