If it fits, why not ?
It is not trivial, but I think, It can be done with some attention.
The PS/2 socket could be placed just next to that side where the double metal spacer will be (like in the picture),
even it can be UNDER the universal PCB (between the mobo and the universal PCB) choosing an 1.5 cm spacer height,
and the arduino micro board would (probably) fit next to the PS/2 socket.
The micro board would reach above the spacer at the edge of the mobo,
but I think with an additional lines of IC socket like these:
http://p.globalsources.com/IMAGES/PDT/B1112823448/Pitch-Single-in-Line-SIP-Socket.jpgI can raise the micro board above the screw what will fix the universal PCB to the spacer.
The arduino micro have contacts only at the edges of the board, not in the middle.
So the height of the whole stuff would reach approx. 2.5 cm, but the PS/2 socket would be just 1-2 mm above the EP mobo, just next to the double metal spacer ...
It can be stable enough, cannot it ?