I noticed while playing with Real Scale Boosters that a single RD-180 from that mod has roll control while a single SSTU-RD-180 does not have roll control.
Is this intentional/limitation/oversight or did I miss some setting adjustment?

I don't know offhand if the real engine has this capability, but I would assume so.

I'm not limiting it in the config, so this would appear to be an issue with the stock gimbal module.

MODULE
{
	name = ModuleGimbal
	gimbalTransformName = RD-180-Gimbal
	gimbalRange = 5
	useGimbalResponseSpeed = false
}

If this in fact a problem, there is nothing that I can do to fix it.

Have you verified that the 'roll' control is enabled in the gimbals 'advanced tweakables' options?

This must be an issue with RO, as it works fine in stock...