Firstly, that is such a cool thing to have to play round with. Now I've never done this kind of thing before, but if I was going to turn a piano into a liquor cabinet I'd probably do it like this...
Take the back off the piano
Strip the interior (ie. so the keys and pedals are still in place on the front but the inside of the piano is pretty much empty)
Turn one of the piano sides into a hinged door - based on the way piano lids are, i'd recommend putting the hinges on the back piece and then turning the lid into a permanent structure.
Put shelves inside the hollow - maybe on runners so you can put stuff 'at the back' without having to reach through everything to get at it
Put it all back together again
I can't guarantee that this will work or is even feasible, but if I suddenly had a piano and didn't know what to do with it, I'd maybe try this (although I think mine would be a cd cabinet!)
Good luck modifying