I did not noticed RXTX library here, but rather jSerialComm2.7.jar, which seems to not use native libraries.
Only thing which uses native code, is libUSBDevice.so, used by WatchPower.jar directly.
I installed 32-bit Linux version of WatchPower on x86 Linux PC, copied it over Raspberry Pi, made some modifications to LAX config to not use embedded Java VM (because it is x86 one) and some other modifications, to launch it. Still, for having working USB connection, I written my own libUSBDevice.so replacement, and it works quite well so far.