I checked during my audio system installation, and there's the gas tank...!
Besides, the back seat fits quite close to the floor (top of the gas tank) in the P5s, so very probably, that option is simply out of the question.
I am also looking forward a hidden sub-box installation even if that means lowering my sub size from 12 to 10 inches...!.
So far I found the spare tire well has a protrusion that doesn't allow me to fit a sub box below that tire... (mine is a 2003 P5 without stock sub)
So far it seems I'll have to replace / custom make a sub box in the right side of the trunk, more or less where the jack sits. This will anyway extend inside the trunk some 5 - 7 inches, and I'll have to move the jack near the spare tire (I still don't know how I will secure it...). Still, I expect to have some 0.7 cu. ft. for a 10 inch sub, using fiberglass to make the most out of the curves behind the stock plastic panel...
You could probably fit 2 10" subs in the tire well if they have a normal sized magnet structure (one on each side of the hump). If not, check out www.illusionaudio.com. Their subs will fit anywhere.