Even device's own ground plane some times works. Though, it's near impossible to do with isolated power supplies. Modern ULP MCUs are incredibly power efficient, and can run for a day on a power of a single LED blink.
Anything wireless is still out of the question unless real ground is there though. Without ground, a dedicated power harvesting circuit would certainly provide more power.
We experimented with harvesting, with 10cm antennas, it's just enough to do a single beep once 20-30 minutes using specialised ULP UHF radios, or RX for a few seconds once a minute.
