What would be a solution to keep the VM-3P75CT permanently visible on the EkranoGX, even when there is no voltage at the source to be measured?
An alternative AC source would always be available.
What exactly are you trying to achieve with this?
The readings of the VM-3P75CT are based on current, voltage, and phase angle.
If you feed it other values, it will return incorrect readings.
In case there is no power at the VM-3P75CT, the device isn’t visible at the GX-console nor on VRM because of missing power.
As I’ve severall sources and more than one VM-3P75CT I would like to see the device showing “0” values instead of disappearing at all.
In my opinion, it doesn’t make sense when it operates as a grid meter.
And as already mentioned above, your measurements are no longer accurate during normal
operation.
But if not, just give it a try 
1x grid plus 1x generator port plus 1x generator stb
The problem with your request is, that the grid meter has to be supplied with the same voltage, that it is reading as DirkW already said.
Otherwise the voltage and current will be calculated wrong and the power values will not be accurate any more. That’s why it’s not even meaningful to supply the VM-3p grid meter from some other point in the wiring (e.g. some people connect it after a MCB or RCD).
An other point is, that in an ESS, the grid meter should be “lost” as well in a power outage, because it does not make sense if the MP2 has no voltage on the AC_In any more, but the grid meter is still delivering values…
Got it, thank you!
No specific power supply other than the power to measure.