So, for me to get this right (because I may be very wrong)..:

The aircraft is surfing along the 148,165 hPa pressure level, relative to 1013,25 hPa. This translates to a pressure altitude of 45.000 ft on the altimeter; the indication won't change as long as the outside pressure measures 148,165 hPa. With no change in outside static pressure, there will be no barometric vertical speed.

I do not yet understand this conclusion: “the decreasing lift will automatically make the aircraft descend on the true altitude scale”. Why would lift decrease if performance, air pressure and density remain the same?

For the true altitude [TA] scale we could have a look at the ICAO – International Standard Atmosphere [ISA] to read and use values in terms of geometrical altitude for p, ρ, T and g. Only T is a constant value (216,65K), the others vary with geometrical altitude.

In order to be able to fly at MAX ALT, the aircraft *mass* will be around 200.000 kgs. Let’s move from “high to low”. At the point where SLP = 1000 hPa, TA will be ± 44.700 ft. The ISA value for g at 44.700 ft geometrical altitude = 9,6702 m/s2 - resulting in an aircraft *weight* of 1934040 N.

At the point, where SLP = 967 hPa, the TA will be ± 43700 ft; the value for g at 43.700 ft geometrical altitude = 9,6732 m/s2, resulting in an aircraft weight of 1934640 N. So: slightly more W at the lower geometrical altitude. If Lift is momentarily set to a fixed value, my guess would be: on the TA scale, the aircraft descends slowly because of L, becoming slightly less than W. And maybe the IRS will pick this up, but I doubt it since Δ g is very small (1,0003 over ten minutes = long term). My bet: there will be no indication of Vertical Speed.

Yet another effect will be that this 200T aircraft is consuming fuel; in ten minutes ± 1130 kgs (or 10960 N – Freighter), or ± 1160 kgs (11217 N – PAX/Combi). As Jcomm mentioned, the real atmosphere will deviate from the one formulated in ISA: already on a daily basis, and significantly near a tropical cyclone. Confusing many variables. But…: is has been a long time, since I was at school.

Simon