I've been still thinking how to switch off the filament (my display is always off, but the filament is always on), I will do it with a simple switch placed somewhere on the rear, perhaps..
Btw. - this is what I did in past to deload my PC and save energy - there is the rear db9 serial connector, it has got 2 "free" pins (see the service manual).
I've been using one of the free pin as a 5V source (off the 5V outguard regulator placed nearby, via a 4R7 and 100nF to gnd) for the powering the external mcu.
The second free pin has been used as the output of the internal LM35 temperature sensor (powered off the 5V outguard vreg).
The meter's rs232 to cmos 3V3 level is simply done via a transistor (the bluepill reads the incoming data only).
As I wrote in past here there is the BT HC-05 sending data (115k2) off the bluepill to any external device (a cheapo smartphone with serial terminal in my case) logging the data coming off the bluepill (and I make 2x 16bit temperature measurements, temperature/gain compensation, averaging/smoothing of any kind, stddev, all in float64, incl. time/date capture from DS3231 in that bluepill, sending all the results upon each new sample as a .csv record).
Thus the total power consumption with long measurements is minimal (except the filament)