The oil control ring is indeed the bottom ring (it's really a stack of hardware and not a single ring like the compression rings) on pretty much any piston, this one included.
What's the method of planned application, how are you going to get the Seafoam to the right place? I would think pouring some down each spark plug hole and bumping the crankshaft back and forth a bit would probably work some into the oil control ring stack, is there another method described on miata.net?
I say it's worth a shot since a can of the stuff is a lot cheaper than a rebuild, but I don't know if I would expect miracles.