I know this can be done on Courtyard and some of the other maps, but on the new map Highlands I created a larger area where you can green screen. Its a little buggy, the green color turns yellowish sometimes, all you need to do to fix this is change the team color of the pallets to green and the pallets will be the right color. You can also change the color of the pallets as you desire to key what ever you want. Enjoy
Video: http://www.youtube.com/watch?v=NNV12WT7BZc
Picture: http://img534.imageshack.us/img534/7084/reach15204726full.jpg
Map:http://www.bungie.net/Stats/Reach/FileDetails.aspx?fid=152045 37&player=Ou7of7ime