Using Adafruit's TrinketHidCombo library I've managed to make a "proper" usb Power button. It sends System Power Down event (works fine on PC), but in OpenELEC it only launches Shutdown menu and awaits for user selection. So the first solution will be used. Code, schematics and quick'n'dirty readme are on Github (code and readme in PowerBtn2 directory).

Everything is slowly mounted in it's place. And it seems that I should be able to stuff power adapter inside the case :)

Done
-main components are mounted
-fan is working (I've never used my U2 with a fan before)
-PowerCtrl board
-PowerButton

To do:
-powered usb hub
-custom HDMI cable
-extension cable for wifi antenna
-back panel
-maybe an USB port on the front panel
-finish the front panel (sanding paper, bondo, paint etc)

Video from today's test run, sorry for quality:

And everything except for power adapter is mounted:

Small box near the PowerCtrl board is Medion X10 MCE USB receiver, antenna-grey wire hanging outside- will be covered with front panel only.

And with DVD mounted:

DVD doesn't touch the USB hub, there's about 2mm of free space left.