Not sure we're on the same page Robh. I used meatloaf's piano intro as an example of an accompaniest who can come up with a piano part on the fly for your songs.

Like your a songwriter, u sing and play guitar and dont play piano, and you put down your guitar and vocals, and now you need a piano player to write you a part that compliments your melody and your style.

So you're not only borrowing his ability to play along with you, but his ability to create musical phrases that fit specifically with your song.

It's askin way too much of a software to do that, but I actually write on guitar but often hear specific parts on piano that I can hear in my head but cant play.