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Y'all, I'm kicking myself right now. Solution discovered. I'll record it here in case someone stumbles upon this thread with a similar question.

Turns out, there's a Gradient option under Adjustment Layer! There are all sorts of gems in there that I never bothered to investigate! That's what happens when you become complacent with your knowledge, kids.

Anyway, that handy little Gradient adjustment layer automatically fills the full height and width of the artboard with your gradient - linear, radial, whatever type and color your heart desires. And as with all adjustment layers, you can also change the blending mode. Definitely a best practice for creating full-size gradients in an Action.