render = false

This is allowed in the default section of a ucl coloring formula, but not in a GradientColoring plug-in.

Apparently I have to make my own version of GenericGradientColoring from standard.ucl and put "render = false" in there.

You've hit upon one of the big weaknesses of plugins: there are many options you can't set from within the plugin.

Correct. It should be possible to be able to add render=false to a plugin and use the combined render settings of all plugins in a coloring/formula. I'll look into that.