what i want is to detect an external +5 when boost is active. one idea is to monitor curent sink from battery using bq27200.
enable charge is an option ( manual connection of external source required ) so an button in status area menu is enough

anyway
i found an workaround ( hardware mod ) for ALL hostmode problems. i am glad to share with you

details
bq27200 have an unused GPIO pin, out mode is open drain transistor 5mA sink capable.
by connecting this pin ( 9 ) to R4283 - C4280 - F4 ( isp 1707 ) node we can controll voltage applied to the usb phy. bq chip can sink all curent flowing r4283 to ground ( 5V / 1k = 5mA ) if any piece of code will program gpio pin of bq27200 as input this don't affect normal usb operation
what we get ?
independent source of vbus speed selection. even if external 5V is applied, permanently to n900 we can any time make speed selection by programming pin 9 of bq27200 as output and assert low
this is equivalent to toggle boostoff , booston .
so charging any time or boosting to 5v the bus line the job is freed of any constraint

the link between 27200 and 1707 can be easily done ( no underchip conections )