On a stock N900 isn't the emergency charge mode a very low mA load which engages and gives the steady(vs pulsing) amber notification light? It is when there is so little battery the phone is unable to even boot into a smart charging mode. From what I remember from here on TMO the phone will need something like overnight to just have enough battery power to boot into the smart charge mode and begin normal charging if that even works. Best bet is to get an external charger for these 'jump start' type situations where the phone is left dead and helpless, the OpenMoko GTA02 had a similar situation.
http://www.amazon.com/TOOGOO-Battery.../dp/B00FFYLSE8
I would suggest waiting on any solder solution as that could easily ruin a working USB port.