"The problem I have with furniture like that is..."
A slide out drawer for the querty keyboard that fits just below the piano keyboard is a pretty simple fix for that quandry!Not perfect but certainly viable...
You missed this:
"I always want to have all my gear - mixer, keyboard controller, computer keyboard and mouse - all at desk level
That doesn't do it, nor does Nick's piece.
I've built myself a few desks that have the MIDI keyboard controller directly underneath, in a sliding drawer like that. The problem with that for me is that, with the height of a decent MIDI keyboard and a drawer and then the upper desk surface, you end up with a compromise somewhere. Either the MIDI keyboarfd is too low or the desktop surface is too high.
A true piano keyboard is in fact a little lower than a traditional desktop surface so, I initially thought it would be do-able like that (and maybe I'm just too sensitive to the height issue) but you can't get both to be the right height, with a system like that.
So, I just have a desk w/ the computer keyboard and monitor on it, right in front of me, with the DA-7 mixer on the left, angled toward me, and then a MIDI keyboard controller to my right, and other stuff (MIDI Drums, floor controllers for guitars, etc.) on my left. All arranged in a semi-circle so I can just kind of turn my chair and get to everything.