the thread
http://talk.maemo.org/showthread.php?t=57734

in theory will work.
charger asic will keep the voltage across battery terminals under 4.2 volts and n900 current consumption will keep charger chip active .
the only reason for not working in this mode is bme software
max voltage of twl4030 chip I think it is 4.5 V ( according tps65950 datasheet ) so if bq24150 keep the voltage under this value it is ok for running.
anybody want to try 4V high power zenner diode ?