Submitted into the show, the game lost video part way through and again didn't make it through the show :(
Testing the cab using a portable scope I'd recently bought confirmed that the board was outputting video and that the problem was likely a chassis fault. There was HV but nothing on screen. Since this is a 900E/USA chassis it has single US 120V power input plug and a US pin-out on the video inputs. In order to test it on my existing 900 and 900E setup I needed to make a power adaptor to convert to it. The power input I took off the 900's 128VAC input (close enough to the 120V for testing) and the 240V left unused. I didn't have exactly the right connector but something close enough to use with a little bit of pin socket widening. The video input adaptor I made out of some reclaimed yoke connector parts.
During setup for testing I also realized that the 900E/USA CRT socket didn't fit my test tube (an MTC-900 Videocolor A51-421X). Using a sharpie I carefully removed the hot glue seal on the pin guide to remove the guide and then was able to attach the 900E/USA neck board onto the bare pins for testing. The picture was very dim but came up to something not too far off normal with brightness and contrast both turned up full. There was also a slight ripple across the picture. Having never tried a 900E/USA on this type of CRT before I couldn't be sure if the chassis did have something wrong with it or it was not quite compatible with this CRT.
Trying the chassis back in the cab on its intended CRT confirmed the chassis behaved exactly the same way as on the bench - the picture was still slightly too dim with full brightness & contrast and there was a ripple across the picture. The picture was also smaller than it should have been. Taken all together I suspected a power problem.
Back on the test bench (again) attention turned to the power regulator circuit. The B+ on this chassis was supposed to be 115VDC but measured as 97VDC. The B+ adjustment pot had no effect on the voltage. Nothing much on the regulator circuit matched the values on the schematics and the 220R resistor was getting very hot. I didn't fully understand how this circuit works so I went with standard transistor testing throughout the power section. None of the back-to-back diode checks found anything obvious in circuit but removing all the transistors and testing them in the multi-meter found TR7, a BC547, was bad. Replacing it fixed the power regulation and the picture was back to normal. Since I'd been messing with the B+ pot I needed to also recalibrate it back to 115VDC.
With the chassis back in the cab and after resetting the brightness & contrast the picture was back to normal and looking good ready for the next show :)