i remember seeing an article in re/p (the magazine) back in the 80's about a studio that had been built around a piano, by a singing pianist. he set it up so that the keyboard was in the main room, and the rest of the instrument was in an iso booth. obviously, there was still a bit of leakage, but it worked for him. a few years later, i saw a note that he had rebuilt the studio and no longer had the iso'd piano.
i did a singing pianist session this week. when we needed a fix, we had her play and sing the required section again, then edited it in. not perfect, but pretty believable. the reason we were doing this recording is that she wasn't happy with the original recording of the song... which had included an entire band, and didn't match her intended feel. the way to get what she wanted was to play and sing together. so the technical limitations were overruled by the desired feeling of the performance.
i didn't find any problem with getting an "acceptable" amount of leakage in this case. the piano still had a reasonable stereo spread, and wasn't too woofy from leakage into the vocal mic. but that depends so much on the performer's dynamic both vocally and on the keyboard... woe to the engineer whose client has heavy hands and a whispery voice....