C64: PAL 0.985 MHz, NTSC 1.023 MHz
VIC-20: PAL 1.108 MHz, NTSC 1.02 MHz
I think the PET nominally is exactly 1 MHz, so in theory the NTSC setting for either C64 or VIC-20 should work with your PET. However I've never tried it myself so there might be something else that causes problems.
It appears WAV-PRG v3.6 was modified/released on September 7, 2011 so there should still be hope for improvements. As far as I remember looking at the source, it should just be a matter of adding timing constants and a bit of GUI/CLI parsing code to get PET tapes to work, assuming they really are encoded in the same way as VIC and C64 tapes.