Piano Mark,
I too love the piano and have found BIAB to be a great accompianment. I have found it easier to input the melodies with a small keyboard controller (USB). Just press record in either BIAB or Realtracks after you have entered the chords in the "style" of your choice, set your key and tempo. You will surely want Real Tracks Disc #70 with the Piano Solo accompianments. They are so beautifully done and a great choice of styles.
Of course guitars and strings are always good with piano too so you have a great variety to choose from even in the midi instruments. After you have completed your song, you can always mute the melody part and play along live if you choose but you will need the melody there if you want to add the note based lyrics for onscreen reference. There are several pieces I have done using these acccompianments posted on my site at the link below if you would be interested in hearing them. http://soundclick.com/share.cfm?id=11489803