There is a lock washer, but it's not very effective. The nut needs to be very tight and it's hard to grab it with a wrench. As you can guess, I broke the connection, but my poor soldering skill was good enough for the repair.
I have not tested the HD-SDI input fully, but I will soon. You should not expect an improvement in image quality using HD-SDI vs. HDMI for BD. The advantages of HD-SDI are very long cable runs, compatibility with SDI (HD-SDI) input processors, and no HDMI handshake issues.
A note of caution regarding the SDI (HD-SDI) BNC connector on the Host. It seems to have a tendency to become loose. If you feel any rotation at all, stop and open the unit to tighten the retaining nut very tightly. It's very easy to do and it could be a good idea to do it before the connector has a chance to loosen. A rotating connector will break the short internal wire connection.
Once you get past the limitation of the vertical offset, there are a lot of options. I prefer one of the Barco DCI models, especially for a theater like this close to expert support, but there are other choices.
The best solution to the vertical offset problem is to introduce some tilt and keystone. The DCI models have angled masking that will produce perfect image edges and a slight keystone is not noticeable in video if there is no edge reference. Although my projector position is well within the vertical offset range, I prefer to use less offset and a slight tilt. The overall image quality is better by keeping the lens closer to the on axis position.
What's involved in taking a photo of a G90 CRT to show phosphor wear? Is it enough to just remove the lens without doing anything with the liquid coupling? Does the lens simply come off after the retaining screws are removed? Can this be done easily with the projector ceiling mounted?