Easy way of curing flat battery is also old nokia dumbphone. Most of the old nokias use same or same voltage battery as the N900. You could easily use the dumbphone to charge the battery a little and so that your N900 can initialise the charging circuit. You just have to be sure that the pins are in the right order in your dumbphone battery compared to the N900 battery (+,T,-)

I used old dumbphone for a while to charge my batteries, when I had my USB broken, and still use it to charge my reserve battery. The pins were in wrong order, so I took some wires and made a cradle out of plastic and routed the wires there in right order to charge the battery easily.