I also thought about a way to do it, by temporarily putting Roy's and Mewtwo's Star KO sounds over any other character (that has the filter) Star KO, and play the SFX on the Sound Test menu, and proceed to record the sounds (now with the filter) either with any recording app while using an emulator or with a recording device (without emulator), then, the last step would be putting the new pseudo-filtered sounds on the respective sawnd files for Roy and Mewtwo. Though this task
may be quite tedious, and i'm unsure about how the SFXs quality could end up.
Either way, you're the SFX Lord here, and you're the one who will choose.