Some more details on that would be great.
The hole, even with a fan added, looks like it's in altogether the wrong place to help with cooling. If anything, it would disrupt the designed airpath - cool air drawn in there will totally bypass the logic board, resulting in less airflow over the CPU and mainboard, maybe even none at all. That's begging for sealing up with a LED-lit Apple logo, imho
Any airflow improvements should be designed to help air pass along the designed path, not to compete with it./ETA/ On the other hand, I could perhaps see how a fan in that position blowing out, rather than in, might help.