How can I build a RGB+CSync to SCART video cable for a vintage camera control unit?
OK. I figured out how to do it. Here's my solution for anybody else who's interested in achieving this (although it might actually be just me :-) ).

I cut a three-cable solid-core RCA component video cable in half, leaving me with six RCA plugs plus cables. I used four of the cables, one each for R, G, B, and CSync. I attached the open ends of the cables to SCART pins 15, 11, 7, and 20, respectively. I cut one end off a USB cable and attached the +5V wire to SCART pin 16 (composite/RGB selection), and then made sure to connect all the ground wires on both ends to ground. I connected the RCA ends of the finished cable to the CCU with RCA->BNC adapters, connected the USB plug to a smartphone charger adapter, and then connected the SCART plug to the SCART->HDMI adapter, and finally got a (nice-looking!) RGB signal from the camera.

I might go back and add a 100 ohm resistor on SCART pin 16 to bring the voltage down a bit (the max voltage needed to select RGB with SCART is 3V), but my setup works as is, so maybe it's unnecessary.