Wow, it's been a while.
Recently I dug out my N900 and started messing with it, I soon found out that the battery has lost almost a half of it's capacity.
With not much to loose I gently pried off the top plastic from the battery where the connector is. Then I carefully cut off the metal tabs holding the plastic (and the protection board embedded) to the actual cell. Be careful not to short any of the tabs to the casing when cutting, or be ready to scram
After that I connected a USB cable through a regular rectifier diode. USB is 5V, while a fully charged battery is around 4.2V, so a 0.7V drop across the diode puts me in the ballpark.
With that done the N900 boots right up.