Powering a ESP8266 that needs to be mains-referenced?
You can use any power supply you want (transformer, USB cell phone charger...) provided it delivers the proper voltage and current really...

Of course this supply will be isolated from mains, but that doesn't matter, since you will reference the secondary to mains via a separate connection through the energy meter part of your design.

Also an isolated supply is very useful if you want to program the micro or  connect it to your PC to run a debugger for example. Can't do that with a capacitive dropper. So make sure you have a convenient way of removing the connection to mains on the energy meter side (like, by not plugging it in).