At least a few years ago, there was a guy in the Netherlands selling adapters--basically he was carving the shells of old batteries, soldering in a resistor, and leaving space for the new battery inside.
This is why, when I bought my own Luna Pro, I went with the Luna Pro F, even though I didn't really need the flash meter. Takes a regular 9v battery.
More likely a diode rather than a resistor. And some models of the Luna Pro SBC (no flash metering) also take the standard rectangular 9V battery.