Mr. Fulmer,  My advice is to get the powdery residue out of the piano
immediately if not sooner.  Rob Goodale's method of removal is fine.

The reason I say this is because the agent in dry chemical fire
extinguishers is very corrosive -- when used to extinguish electrical
fires, all wiring must be replaced due to the corrosive qualities.

The working agent in these extinguishers is potassium bicarbonate of
soda, and the potassium etches metal.  Whatever you do, don't let any
water mix with it because that will enhance the corrosive action.

Best of luck,

Dan Armstrong
Armstrong Piano Tuning & Repair
Pine River, MN