But the BOOST gage - am I correct in perceiving it as a "PSIG" instrument, as opposed to an absolute pressure device?
No, all gauges measuring induction tube pressure are measuring absolute pressure.
An extract from 1943 | 2839 | Flight Archive
A boost pressure gauge, in spite of its name, differs fundamentally from the fuel pressure gauges, oil pressure gauges, and other pressure-measuring instruments used in an aircraft. Like an altimeter, it records absolute pressure, and not merely pressure above atmospheric. In fact, the American name of "manifold pressure gauge " is more correct, the gauge giving no indication of the extent to which the supercharger has boosted atmospheric pressure.
The orthodox British system of boost gauge calibration ignores this fundamental fact, pressures being given in pounds per square inch above or below an absolute pressure of 14.7 pounds per square inch. The latter pressure, which is the mean sea level atmospheric pressure, is arbitrarily known as "zero boost."
The fact that the boost/MP gauge reads ambient pressure prior to engine start is used by some manufactures as a power check on the engine run up before take off. An example from one particular aircraft.
Note MP prior to engine start.
Run Up - Adjust throttle to field barometric pressure as read on MP gauge before starting engine
For propeller type XXX check for 2150 +-50 RPM
For propeller type YYY check for 2250 +-50 RPM
If RPM is too low for given MP, engine is not developing sufficient power and should be checked before flight.