I dont think there is too much to worry about. If you want to keep an extinguisher handy a dry chem type ABC or even a BC would be sufficient. Personally I prefer a CO2. The BC type is for flammable liquid
and energized electrical fires. Potable extinguisers are for fires in the incipiant stage and are only practical as a first line defense.

There must be a significant amount of magnesium in the TCH for Tyota to reccommend a class D extinguisher. Some components like the seat springs may be made of magnesium. If you need one, most likely the majority of the car would be involved and a portable extinguisher would not be effective. If you have a fire it is most likely going to be in the engine compartment.

Most fire departments don't carry a class D on the rigs. They are seldom needed. Years ago, the VW beetle and older porsches would catch fire and they had magnesium blocks. Required large amounts of water to extinguish and most had to have an application of foam due to fuel igniting.

The TCH emergency response guide makes no mention of using a class D extinguisher. Here is the link if anybody is interested. Some good information....

http://www.toyota.ca/NWS/media/pdf/07camhyb_erg_e.pdf