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The Merlin engine in the Falcon 9 also uses fuel (RP-1) as hydraulic fluid, I believe.

Indeed it does. It was a notoriously fickle design to get right and for a long time they outsourced some parts of it to a company. One of the few things they outsourced. I remember you could find a listing on the company website for this very specific valve, with wink wink references to SpaceX.

I think they managed to bring it all in-house now.

I wonder how they normalize the flow of RP-1 through the hydraulic actuators. You wouldn't want the mixture ratio in the engine itself to vary depending on the gimbaling. I guess there's a bypass where fluid is just dumped back into the pump inlets without having been used, so the flow is always constant.

I don't know about today, but early Falcon 9 gridfin actuators used an open loop of RP-1 as the working fluid as well. It would drain into the tank, and be burned in the engines on descent.

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