To obtain an S-meter scale to be proud of, you will have to use pen
and ink with a signal generator and 100-dB stepped attenuator.
Attempts to calibrate the scale with clever, highly complicated
electronics will get you nowhere in a long time. And will cost you
more than the remainder of the receiver.

Sorry to be so despondent.
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Reg, G4FGQ
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Just an idea ; if one is really 'dead keen' to have an 'accurate ? '
S-meter readout, you could construct an indicator with individual
threshold opamp comparators ( up to 4 in a DIL package) each driving a LED.
When calibrating with a signal gen as a 50 Ohms source ,starting with
S-9 being 50 microvolts being -73dBm (or 5 microvolts being -93dBm for
VHF and higher) the relevant LEDs can then be set separately below that
level with 6dB steps and above S-9 with 10 dB steps by accepting the
quasi-log voltage range generated by the AGC as fed to the traditional
analogue S-meter
A fancy feature would be different colour LEDs showing signal strength
above S-9. I feel that ,while accepting any 'professional'
comments,this would be a practical 'amateur ' (low cost) solution .

Frank GM0CSZ / KN6WH an 'Amateur'