Played - as in: mock-up - around yesterday night, because I was still thinking about how to add the 280 ON TOP of the case.
So turns out, the 140-to-120 adapter brackets I bought for the K77 Lite would also fit VERY WELL on top of the A4-H2O.
Those would connect to the mounting bracket on top, then attach the radiator with VERY SHORT screws on top of the adapter bracket. Next, one could attach 2x 120 mm fans on the other side, just like regular.
This method should, in theory, also solve the issues with the tube routing.
A slight disadvantage would be the added height, as the backpack I got only allows for a max of 29 cm (maybe 30, but thats very stuffed then). So my next idea was to get foldable legs, which I eventually found in the ones available for laptops. After a bit of digging I stumbled across some that are made entirely from metal (except for the feet protection), which I've immediatedly ordered, and going to see if I can drill holes, to attach them to the bottom of the case.
That'd allow to fold them in when putting the case into any kind of carrying container, and unfold when in "immobile mode"
cu, w0lf.