Add a cello to a loadsheet? You aren't serious? Why, it weighs less than many other carry on bags? What is it that's so special about cellos that they seem to take on quasi-human form when in an aircraft? Blankets and bags in empty seats aren't counted, why should a cello be different?
For every airline I've handled (and I'm talking upwards of 100 airlines) cello's are always added as a weight onto the loadsheet. Cello on a seat is counted as an 'occupied revenue seat' therefore included in the passenger figure, but not the total heads on board figure.
Check your airlines load control manual, it'll be in there.