Installation complete and road tested. All is well. Dial and needle lighting go thru switched power, the GPS runs direct to the battery. Was surprised that the GPS current draw (pre- and post signal acquisition) was just over 100mA, which explains why my battery was so low after sitting for two days. Fortunately, I can break the connection to the battery easily to avoid running it down.
Oh...Speedhut is discontinuing their current gauge models ("EL lighting") and replacing them with "LED lighting" 1 May 2023. They will look different at night, so if you're running a Speedhut/OEM combo and thinking of replacing the OEM in the future, you might want to get your order in. I was also told the face of the new gauge has a slightly different "texture" and will also not match.
On the plus side...no inverter required for lighting!