I forgot to make more pics, but you see what I did and this mod has been done to other keyboards before. So nothing special here.
I used the Bluetooth Keyboard Adapter from http://handheldsci.com/kb
This adapter is excellent to mod.
Behind the 2 holes (cable ports) are now the red and blue status LED.
The little stick is the on/off switch. In the back you see the Micro USB port for charging the battery.
desoldered both led and PS/2 port (to high)
right sight will keep the battary
on/off switch fixed with hot glue only
Not so nice...
Speaker is loud, beeps for turning on and off. But you could both turn off via configuration.

This kind of Deck Keyboards starts always with led on! And it seems, that the ISO variant is not able to FN+0 to turn LED off.
I will ask my friend later how long the battery lasts. Maybe I will put in a bigger one.