I was also looking for this solution, i have a different usecase however. I have a 3 phase grid connection with a shelly em3 monitoring my usage. On one phase i have a solar inverter with battery that only checks one phase to see if the inverter needs to discharge the battery or not. I would like to use the sum of the 3 phases that i get from the shelly to fake the CT clamp to the inverter.
The grid meter just adds up the sum of the 3 phases, so it doesnt matter if i return power on phase1 and use power on phase 2.