The big problem with a Pi is power management. The kernel has no support for any low-power states (and neither do basically any other single-board ARM computers). The Pi 3, when all IO (USB, HDMI etc) chips are turned off, consumes about 250 mA. This includes when the Pi is "off". There is no way to reduce power below this point short of removing power to the entire board via some external means. On a battery the size of those in the N900, N9 etc, this will last less than 10 hours at minimum usage. This is a big reason why nobody has made a proper "hackable" phone from any single-board Linux computer - there simply is no power management solution.