nesdev.org

Is it possible to trigger the scanline IRQ multiple times during a frame, or do I just have to use the IRQ as a starting point for timed code?

I'm still working with some scanline effects and I felt this would be the easiest way to do it.

Yes, you can use it multiple times. You just have to set it for the next IRQ inside each IRQ. Though I think there is a minimum 2-scanline limit, so you can't have interrupts every scanline.