If this is a specific example of what you want to do, you might not need to worry about syncing it to the frame, since (with frames being 1/30th or 1/25th of a second) it’ll likely be perceived as an overall reduced brightness across all the frames.
If you’re looking for slower timing than every other frame, it will probably be more perceptible. In that case, you’re going to want something that outputs a frame clock. The LZX Cadet 1 Sync Gen can do that, though it’s out of production and I think some of the parts to build a new DIY one are hard to find now, so you best bet is to find a used one if this is what you want. The LZX Visual Cortex can do that too (and do a lot more neat stuff), but it’s out of production now too (and pricey if the frame clock is the only thing you want it for).
Some people (including myself) were discussing making a module that would work with the LZX Gen3 line (and as a result, would also work with normal CVBS) to output a frame clock and some other clocks from the signal, but I don’t know if any of us actually completed the project. (I unfortunately got waylaid by a lot of things, and shifted my focus to some other projects. I may yet revisit it one day, but it’s not high on my list right now.)