I have a Pretech caliper on my Commando, I saw it on a bike featured in a magazine and liked it because there was no adaptor plate. I thought the performance was disapointing, however when the first set of pads wore out and were replaced by some I got locally, the performance improved markedly, the originals may have been a bit sporty for my riding style.
I actually wondered about looking into getting one made to fit onto the girdraulics, they arent toofor from me so it might be worth a trip.