Paul will throw his hands up in horror at the suggestion, so I’ll say it quietly, I’ve installed PlatformIO on a Pi and successfully compiled the sketch & uploaded the file to an emonTX via the serial interface, so it should work for the emonPi. EmonTX to Rpi - Direct Serial Connection - #48 by borpin.
A word of warning, on the emonPi by default, the firmware is uploaded on update. You can select not to, but you have to select not to.