[pkautzsch]

Under the piano: I once used a KM183 (I think, but don't really remember) in quasi-PZM somewhere in the middle (though not exactly!) on the floor. The room was quite reflective, so I did get enough attack, and it would have sounded really too reflective with "normal" positioning on stands (that's why I tried quasi-PZM).
On another occasion, I had a KM184 about less than 1 ft from the body under the piano in a live recording of some Gershwin orchestra+piano+vox. Had the piano quite good on the main, but it sounded too "classical". That very close miking gave an air of "jazziness". Very direct and dry, but not really transparent since there is almost no attack. You just get the sound of the frame, and no direct sound from the strings (of course!).
In your context with a small band I'd maybe use it as an option, but also put up a cardioid facing the strings at the hammer end, and maybe another one at the tail end.