Can a power valve "open" at idle? Yes, if the vacuum is below the PV rating.

Can it provide additional fuel at idle? No, as long as the motor is idling on the idle circuit.
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The power-valve circuit is completely separate from the idle circuit. If you have less vacuum at idle than your power valve is rated, then yes the power valve will open. BUT, the engine will not receive any additional fuel until the main circuit begins to flow fuel out of the boosters.

If your engine is idling high enough that you are no longer on the idle circuit, and you still have less vacuum at this rpm than your power valve rating, then you may be feeding your engine with additional fuel thru the power valve.