Yep I'm from Germany too
In total I've wired all buttons to the D-sub connector: 16 lines including Power-On, Reset, Ground and V+.
But for a portable design, you could only connect the four directions and six to eight buttons. The good thing is that all buttons share the same ground (as you assumed), that way it's pretty easy to do the soldering.
Unfortunately that would mean to hack the pad as well and create another D-sub connector. Also you would need to find better solder points than I did, if you want to keep your dingoo's buttons. The way I did it is a one way ticket.
But I agree, an easy to use external controller (like the A330) or an USB Host Port would be awesome