So, while I work on getting the tilt to work, with vislaw's suggestions, or by other means, here is a little more detail on what I have done.
Installing the Zenmuse was very straight forward and I just plugged into the Zenmuse port in the Vision's board. I bypassed the PMU and GCU all together and it's all good.
In the meantime of course, I have no FPV, and while there are ways to add that, for now I rigged the Vision's camera to act as a fixed FPV so I can have the video and telemetry as the GoPro captures the good stuff from the gimbal.
Here are a few pics of the install on a 3D printed bracket. I made a couple of iterations of it and I'm sure I still don't have it quite right. Next I'll also print an enclosure for the camera.
Sure, the Vision is really not meant for this, and there are better ways to arrive at a gimbal setup with FPV, but this has turned out to be a very fun project. Yet, it really irks me that DJI would restrict access to features thru the firmware. They got my money, I already bought the Vision and the Zenmuse from them. I don't know what they gain by restricting those of us that might want to tinker with it.