You can put a monitor on top, like an ACD or something. BUT, the antennae of both the blue tooth and wifi are located just underneath the plastic top, which is why the top is plastic in the first place, to allow signal transmission. If you put an apple monitor on top, with the thick aluminum base, you'll almost completely kill your wifi and bluetooth abilities.
That aside, the mini has a hard time keeping its processor cool already (hovering around 80 to 90 for dual core under load), not sure if you might add a few degrees to the CPU by covering the top plastic, and reducing the mini's bottom clearance for air intake (held up by what I believe is rubber damping), you might choke your mini's cooling abilities a bit.