In The Field Hotswapping:

You need a dongle *) that has two slots for contacting 2 BL-5J batteries, a 2way switch, and a 1000uF 6.3V capacitor. And a USB-A-receptacle with D+/- short plus CA-101, or a charger-cable stub.
[edit] Depending on component variance this might not work with voltage of empty battery, so some sort of booster might be needed to step up voltage of even an empty battery to a reliable V-IN(min) level of >4.0V for BQ24150

Procedure:
Plug in fresh battery to dongle,
Set 2way switch to connect cable/receptacle of dongle and capacitor to the slot with fresh battery.
Plug in dongle to N900.
Do the hotswap procedure as described before, 1st part, to remove weak battery.
Insert weak battery to dongle.
Throw switch to connect capacitor and receptacle/cable with weak battery.
Remove fresh battery from dongle and insert to phone.
Finish hotswap procedure (2nd part)
Unplug dongle


*) [edit] just for a few bits of info to base my decision on: how many of you were interested in such a dongle? I might offer DIY kits with prepared components (I'm NOT going to sell complete devices, with warranty and whatnot)


jOERG