The gauge has three terminals:
G
+
┴ (looks like an inverted "T")
The wiring diagram shows three wires "B" "G" and "WN" which I take for "Black" "Green" and "White/Brown" Am I correct to assume they should be connected thusly?
G --> Black
+ --> Green
┴ --> White/Brown
I've connected this way and the needle jumps to zero when the ignition is on, but the gauge does not show pressure when the engine is running
And yes, there's pressure. 
The boost gauge will be replaced by a mechanical VDO unit that looks pretty similar to the originals and should be a drop in replacement for the autometer mechanical gauge. I'm going to try to remove the lens and make a new face that will will match the look of the others and allow positioning gauge so the needle is horizontal. I should also be able to get rid of an unnecessary oil line and remove the oil transducer that's mounted where the boost transducer used to be.
Smaug came with a set of three autometer gauges installed but with white-on-black faces. Most folks used white background autometers to compliment a change to white gauges in the main cluster. To me, the Autometers have just never looked "right".
or 
and I guess G is for gauge so swapping them over is correct.
Yep, that was it. I've added a bit to the wiki to save someone else the trouble.