I want to draw some automation for a parameter in an effects plugin and I want to have the parameter values change/cycle gradually over the course of four measure cycles, or eight or sixteen or whatever, using, say, a sine curve. Documentation says I can make the curve follow the snap settings, but problem is: drawing shape only goes up to one measure in length, regardless of how long I set snap. I can only set snap longer than a measure, by the way, by using seconds or ticks (or frames or samples)--which is a bit weird but okay--and PRV honors those longer values when placing, say, midi notes.  Automation drawing shapes do not go past one measure. Does anyone have an alternative way of drawing or painting or copying in "long form" automation curves for a plugin parameter? An alternative, that is, other than drawing the automation by "hand" (mouse) or recording controller movements.

Depends on the shape you want. Fast and slow curves can run between nodes as far apart as you want, and a combination of fast and slow curves can be used to create a sinusoidal curve. Once you have one cycle, you can copy-paste additional ones.

I do all curves by hand and copy them.
just insert a node at the beginning of the measure and another note at the end of the measure or where you were talking about. right click on the line and choose your curve.
 
then you can drag select in the track / right click copy = you will be prompted with a pop up window asking you what kind of copy you want. select only automation (no data or markers).
then paste where you want the repeats.