I would say...since you are using it as a special studio piece, like you would vintage amps and whatnot...it's on your dime to keep it in tune and operational...but I would always ask the artists first if they really plan on using a baby grand or not, before having it tuned up for the session.
Then...I would add it to the bill in a general sort of way...and not as an individual, itemized expense.
IOW...if you have a lot of great studio bonuses ...like great amps, guitars, etc on-tap...I would think that would automatically allow you to charge a slightly higher rate *in general*...than some studio with "0" bonuses/great gear.
I'm not sure how much commercial studios will itemize stuff like that...???
So how friggin big is your entire studio space if your live room is 2 stories high and 100x100???