Hi Hal,
I can't envision a design in which a shorting plug applied to an RCA **input** would cause any problems. Consider the fact that the essentially zero ohm impedance presented to the input by a shorting plug is not greatly different than the very low output impedances (e.g., 10 ohms or perhaps even less in some cases) of some components that might be used to drive that input.
Best regards,
-- Al