Alright... of course all warnings etc. from the initial post apply as well

I connect the whole shebang via USB. Though I am not using the USB port but rather the pads beneath the battery (see the first image).

I took the Powermat receiver of an Iphone 3G Powermat set. You can pretty easily break it off the casing without damaging it. Furthermore, it is internally connected via a flat ribbon cable which detaches easily.
On the second picture you can see where I identified GND and +5V (actually you get something around 5.2V from it). I accidentally fried the first GND soldering pad located next to the +5V pad so I used another GND pad which can be seen in the background.

In the third picture you can see the cabling on the pads beneath the battery. The fourth picture shows the inserted battery and the already modded back-cover including the Powermat receiver and the cabling.

Note: You need to short-circuit +Data and -Data of the USB port in order for the charging to work. Actually I measured something like 2.5 Ohms resistance between these two but short-circuiting also works. For more information see: http://talk.maemo.org/showthread.php...818#post997818
I do this by using a dip switch which connects +Data and -Data.
So when I want to charge via Powermat I have this switch turned on (default for now). One only needs to remember to put this to off if you want to connect via the "normal" USB port. Else you would short-circuit your computers USB port.

Also note that I put a small diode (something above 5V, 40 V or so will do with 1A) between the Powermat +5V and the USB 5V on the N900. The reason for this is to protect the Powermat receiver from the 5V when a USB cable is connected.
I actually don't know if this is needed. I just wanted to make sure I do not fry my receiver the next time when I connect my N900 to my PC via USB.

Well, after this was all setup I simply put the stuff together. Though, on the side where the cables come out from beneath the battery the back cover does not close completely, i.e., there is some opening left. This is no problem for me as I use another cheap silicon case anyway.
As you may have noticed the Powermat receiver nicely aligns with the silicon case. You can see this on the second picture in the first post. You also see the dip switch there.

The whole modding of the back-cover required quite some force. I used a drill to get four holes in and a small saw to cut out a square (smaller than needed). Then I used a cutter knife to carefully increase the created hole to make it fit to the Powermat receiver.
Note: I kept checking the size of the hole each time to ensure I do not cut off to much. I also cut out another small square beneath the camera opening to have a place to put the dip switch and the diode.
Then I used some epoxy glue to attach the receiver to the back-cover. I also used epoxy glue to fixate the dip switch and the diode.
Note: make sure the whole thing works prior to doing this. Also be very careful with this glue stuff as you can easily glue together parts you don't want to be put together permanently, e.g., the back-cover and the main body of the N900.

As you may notice I also cut off some parts of the camera opening. This was necessary as the Powermat receiver is quite big. Though, if you don't cut off too much the camere still works. Even the camera app launches automatically and the pictures still look fine.

Edit: also note that for some, currently unknown, reason the charging just stops after a while.
When you take the phone off the Powermat sender and put it back on the charging continues.
Currently I need to do this about two or three times to get a the N900 fully charged.