I don't have a firm answer to your question, but my guess is that for the player, such an arrangement might result in standing waves and/or interference at certain frequencies with unpredictable effect. How much that matters, of course, depends on you.
In our case, shoving a 9' concert grand into an essentially 10' x 12' room has yielded some funky acoustical properties, as one might expect. There are certain notes whose volume or tone quality will change depending on where my head is located while playing. There was one note in particular that sounded quite different from the surrounding notes, and I thought it might have been a voicing problem. Out of curiosity, I pulled up a live spectrogram and saw that the fundamental tone was entirely missing for that note. After opening the lid completely to investigate the issue, the note was suddenly back to normal, with reappearance of the fundamental. It turns out that having the lid at half stick cause some strange interference pattern that was cancelling out the fundamental.
Minor issues like this are probably inevitable when putting an instrument in an enclosed space. But for me, it doesn't matter - I love the instrument and am very thankful to get to play a concert grand. Plus, we're moving in a year so this is only a short-term arrangement. For those reasons, I can happily overlook the funky acoustical issues and enjoy making music.
