I'm still considering that house fan idea...
If you were to secure a large ventilation filter to the back of a high powered box fan with plastic (for air tightness) and duct tape, secure the fan to the PC case with an airtight plastic tunnel (a modified garbage bag and duct tape should do the trick) and remove the opposite side panel for airflow, you have a pretty slick cooling mechanism. The positive airpressure environment would prevent dust from recirulating into the box, or at least that is the theory
So, assuming the airflow is directed through the filter properly, and aside from the noise, is there anything that would prevent this from working?