Mr. Fulmer,  Not being an expert on extinguishers I couldn't say what
damage the chemical contents are capable of, perhaps nothing.

To be on the safe side, however, I would get it cleaned out at once.
If there are any corrosive properties involved at all it may result in
damage to the strings, particularly the bass windings.

As a full-time piano technician I clean a lot of pianos.  I have my own
methods down pretty fast but here are a few tips.  First I'd remove the
action.  Be careful pulling the action out, DO NOT make the very common
mistake of holding down the end keys or you'll find yourself repairing
broken hammers.

If practical remove the lid.  Sometimes you can just roll the piano
next to a wall where lid can then be leaned all the way open.  Be sure
to cushion it to avoid scratches.

Begin with a vacuum and a small paint brush.  A 1-1/2" or 2" natural
bore's hair brush is a popular favorite.  Clean up as much as you can
including the keybed.  Give the action a good going over while your at
it.  If the action is covered in the powder you may need to remove the
stack (action frame), to do a more detailed job.

The soundboard is the tricky part.  Traditionally technicians have used
a long flat springy thing called a soundboard steel.  The end has a hole
in it to thread a rag through.  You then slip it under the keys and get
busy.  Care must be used, as the soundboard can be scratched easily.

A much easier and better tool is now available from Spurlock Specialty
Tools (707) 452-8564.  This simple device has a felted flat head which
is fed between the strings and then dragged across the board to pick up
the dirt.  It does a fantastic and fast job.  You could probably make
one yourself, (I make my own now), but unless you see one first it
would be tough describing it.  A set of three sizes in a package is
about $20.00 I think.

Once you have the bulk of the piano clean you can use the blowing side
of a vacuum cleaner to get that final little bit hiding in the cracks.
An air compressor is a fantastic tool for this but is not always
practical because of the huge dust clouds they can make.

In your case, however, compressed air might be a good idea so that you
can blow the windings on the bass strings out real good.  It can also
be great for blowing out the action.  Just beware that the cleaning
lady is going to have a fit when the whole church needs dusting
afterward!

Hope this helps,

Rob Goodale, RPT