sorry, that it took so long, I wanted to make pictures before I post. Pictures come later, now some info on my hack, because this is a real cool community which helped me a lot:

I soldered 4 wires on the pads (D+, D-, GND and 5V), about 5 till 10 cm long, on the pads of the N900. The device has to be disassembled for that, I couldn't solder on the pads as it was assembled (maybe with a very small oldering iron it could be possible).
I put the wires through the holes by that you could usually access the pads if the device is put together (and we need it to be together.. )
As mentioned there was no working connection without the battery, so I had to put the battery back in with the wires going by it. That's why you have to use really thin wires.
After putting the device halfway together, at least the battery must have a connection, I connected an USB cable, which I cut on one side, that it has just the plug for the computer and wires on the other side, that where temporarily connected to the wires on my phone. Now my PC recognized a new USB device, rest is flashing, see in the wiki.
Those wires are still on my phone, I'll keep them until I have a USB socket mounted again.
The back cover still fits, a bit harder than usual and I open it regulary if I swap the battery, charged at a wall wart charger.

Mind that 5V and GND or any connectors of the battery should never be shorted!