You can use the UTEC with a JDM ECU, I used one for a little while before I had the ECU reflashed. You need a JDM ECU specific version or else it'll throw 4 injector codes. They don't affect anything performance wise but the JDM version will get rid of the codes.
To put it simply, whatever methods are available for the US WRX are available for the JDM ECU. If you want to use AVCS though, you need to use something that piggybacks off the JDM ECU or supports AVCS like the Hydra or Motec.