The foam would need to be particularly sticky or it would not adhere to the aircraft skin due to the slipstream. A sticky foam was developed in the use to stick to the sides of buildings, even so it still blew off in 50 knot winds.
Most FF foams are slightly toxic.
You could use water from the potable supply to supplement your stored supply.
The Co2/Halon engine fire gas bottles could pressurise the system, but it would take them out of action to help extinguish an engine fire on landing.
Excuse my impertinence, but if it isn't too late to change projects
an onboard fire fighting system would have been handy a few weeks ago when 5 fires were lit by an arsonist, the crew apparently exhausted all the onboard portable extinguishers and resorted to using bottled water.
What is needed is an outlet by each loo where a hose and nozzle can be connected. Hose and nozzle can be locked away and under crew control and deployed when needed. Include a wand that could be inserted into hard to reach places and spray a light mist.