So... I'm not sure why I didn't figure out how to disable the scan velocity modulation earlier, but since I had been looking through the service manual the past couple days, the solution really just jumped out at me. It's really kinda sad that I've watched this set all these years with it enabled, when it is such an easy fix. It is very obvious if you just look at the schematics how to disable it.
Anyways, here's what a checkerboard pattern looks like with SCAVEM enabled (notice the black squares are connected, and the white line on the left of the white squares):
To disable, remove the back cover of the set. Then, on the CRT panel (connected to the neck of the tube), there is a small 3-pin connector on the bottom towards the left (when looking from the front). Just disconnect that connector. This will just disconnect the SCAVEM coil.
Here you can see that the 3-pin wire connects to the white coil around the neck:
And I just tucked the cable under a clip:
And, here's what the same pattern looks like with it disabled (notice the white square corners touch, and much less of a white line on the left of white squares.):
I'm sure that this would have been much more useful many years ago, but, oh well. Really hard to believe that I didn't see this earlier.
While I was in there, I also fixed an issue with my AV1/CVI component input. That input was really sensitive to the signal quality, and low quailty cables or a component selector/receiver would degrade the 1080i signal just enough to cause a really weird picture tearing. Looking at the schematics, the CVI input has a longer path inside the TV. There is a jumper cable from the main board back up to the AV4 board. The AV4 board selects between CVI and AV4 and sends the output to the video processor. I touched up the solder points for the connector for this cable on both the main board and the AV4 board, as well as the AV1/CVI port connectors. This seemed to make the threshold for the tearing less (I only saw it once for a split second during my test material, where I'd normally see it all the time).
Now, the only thing I think I'm missing on this set is HDMI, and I'm not sure I want to spend $200 for an HDFury. I also need to go back in and recalibrate the geometry and the grayscale, but for now, it's looking pretty good.