I dont know how it could be done. But an idea is to: In the engine holder (plastic) at the bottom have 2 pads, so when I insert the engine in the holder, it makes contact to +/-.
Then the use of cables to the engine would be obsolete. What about that?
Interesting idea, has a disadvantage though: as of today the arms of the PCB do not contains any leads, so if they break you only have to find a way to fixate them again, you do not have to find another way to make the motors work again (since the cables flex). If you'd make the arms part of the circuitry to connect the pads you mentioned to the board, that advantage would be gone.
Also: AR.Drone 2.0 (RC-enabled thanks to self-soldered MiruMod) and Hubsan X4 H107
My feeling is that there is a real need for some kind of connector for the motors. Something facilitating field replacement of motors would be awesome. It's way too big of a deal to replace a motor at present. It's not good for the board either. Seems like the design could afford the weight!