Hi,
so alright I invested a few bucks in the idea. aeon_light is right, the jaf-cables do not connect to the data + and data - pins as shown in picture 1.
They can be used however as "fake battery" (box-picture attached) even if the wiring is a bit weird.
I cut off the RJ45-connector and labeled the cables accroding to picture one (box-cable-picture attached).
The wiring in an usb-cable is pretty clear. I labeled it as per defintion (see usb-cable picture).

Before I cut open my Nokia-Charger I wanted to discuss some things.
First problem, since the pins of the box do not connect to the data+ and data- pins from picture 1, but to the mbus-port I think I won't be able to flash (I may try though - n900 is a brick in the moment anyway.)
Second thing is, that the battery wiring in the box looks a bit weird. While the plus-pin has its own cable soldered on, the minus-pin and the gnd are connected with a resistor. Furthermore there is an additional resistor connected between the gnd-pin and its cable. Maybe someone can bring some reasoning in it?
As far as I can see it I would have to connect the minus-cable from the charger (I really do hope it is colorcoded aswell) to the minus-part of the box-cable, do the same for the plus-part and connect gnd and gnd?
Then I would connect the d+ (usb-cable) to the TX-Pin (box) and the d- (usb-cable) to the RX-Pin (box). At least if I go after this post. It would sure make more sense to connect it to the d+ and d- pins of the phone, but that's not possible (the box does not offer any connectors).

Picture 1:


Anyway I will stop at this point and see if smb got further ideas...

So far, blck