Powerbutton works fine on Ubuntu without a battery (don't have one yet). The powerbutton has some basic functions as it is connected to the PMIC, like power-on and hard power-off if you keep it pressed long enough (>10s iirc). Besides that it generates an interrupt on the A64 an then it is completely up to the software (drivers and userpsace) what happens, so your experience varies according to what kernel/os/desktop etc you are running.
Come have a chat in the Pine IRC channel >>