Its a good idea, for more details there is
www.diyaudio.com projection section. There is commercial LCD's produced for this purpose. You shouldnt use a normal LCD that you love an cherish unless you provide adequate cooling. LCD's will fade at high temperature, and too much heat will turn them a deep black (although they usually recover). Either raise the LCD slightly and blow air underneath, or use a metal halide ohp (you still might need cooling even then). Halogen 400W OHP will make this job very tricky, as they give off unbelievable heat waste.
Use a massive fan like wylie's