Hi I managed to get celluloid off a project that I was working with the following method.
I soaked a cloth in hot water (boiled in kettle), removed excess water (cloth placed in colander and used back of ladle to squeeze out excess water) and placed damp cloth over the celluloid.
I found it took a little while to get the first edge to lift but after the glue has softened just keep working on that softened glue edge
Probably what also helped me was the fact there were a few areas where the celluloid was lifting from the edge/ or damaged. This allowed moisture/heat easier access to the glue.
The heat/moisture softened the glue enough to allow me to gently prize/peel off the celluloid with a paint/wallpaper scraper. I managed to remove all the celluloid from the treble end without causing any damage to celluloid or the project (I did take it easy with the scraper). I also used a hair drier at the later later stages (I hasten to add without the wet cloth), directing the warm air between the already loose flap of celluloid and the carcass i.e.directly at the glue. This method also worked but there is probably a real danger of damaging the celluloid surface due to excess heat. Prevent by constantly moving the heat source. (Apologies if that's stating the obvious).
I haven't tried putting the celluloid back on yet. That's a problem for another day.
Hope this helps
John