We bought a 1970's era modular home with fiberboard ceiling tiles. I'd like to lose the tiles and do a slightly textured plastered then painted finish, but since our ceilings aren't quite 7-1/2 feet high now, we don't want to add drywall and lose even more height. Is it possible to plaster directly over the fiberboard, or will it crack, crumble, droop, fall on my head, etc. If it's possible, what products and procedures are recommended?

Plaster on the fiberboard and it will do all the things you don't want, don't do it. Remove the fiberboard and add drywall losing only about 3/8" in height.

Remove the fiber board and rerock it with nosag 1/2 sheetrock or 5/8 sheetrock tape the joints finish it and texture