I had some surprisingly good results recording a concert grand piano in
a NY apartment. The owner of the piano was a collector that rented her
instruments to many artists that recorded in the halls in the NYC area.
The artist I was recording could not afford the hall and moving/tuning
costs so we did it in the apartment. One recording was done with a pair
of Sanken CU-44x cardioid microphones and the other with a pair of
Sennheiser MKH-40 cardioids. There was definately no need for room
mics. The results were quite good. My working distance was 4-5 feet
diagonally out from the soundboard. The instrument was a NY Steinway
concert grand with a gorgeous tone. The NY steinways are generally less
bright than the Hamburg variety.

My recording teacher (from a class I took at Juilliard), Tom Frost,
recorded a couple CD's of Vladimir Horowitz in his living room at home
(an apartment in NYC). He used Sonex to cover the windows and treat the
room. He also used a pair of Schoeps MK2 omni microphones with a Sony
TCD-D10 with custom apogee filters.

So, it certainly can be done with decent results. I would keep the
recording chain simple, possibly treat the space with some carefully
placed sound absorbtion materials, and then place some robust sounding
microphones in a spot that smoothly captures the the frequency range of
the instrument. Some Bosendorfers are rather bright sounding and this
could be an issue with your microphone selection. I should also mention
that I have had much more difficulty getting good results from smaller
instruments such as the Steinway B (@7 feet)

Best of luck,

Mike