Had a chance to play around with the set a bit today. Decided to put the sound issue aside for now, and see what kind of picture I can get through a composite input. Following the RCA suggestion, I injected the video at the output of the video detector. I lifted the plate end of C103, and connected a 75 ohm terminating resistor from the free end to chassis ground. I injected a standard 1Vpp composite video signal across the added resistor. Picture looks acceptable, although the video level is just a touch too low. Cranking the signal up to ~1.5Vpp (or increasing the value of the 75 ohm terminator) gives full contrast range. Video frequency response is somewhat better without going through the IF strip, with details visible into the 4 MHz bar.
Also gave the original 10BP4 a try, to see what it looked like. Image was a bit on the dim side, but I guess that is to be expected with the fairly low accelerating voltage and non-aluminized tube. Certainly watchable in a darkened room. Sweep more than adequate to fill the 10" screen. Electromagnetic focusing coil and ion trap both working properly.
Have a total of 4 10BP4s here to choose from, so will go through them all with my CR31 and choose the best one for final assembly.