I'm all for creativity but I have to say that I hate it when people commit to something prior to having any actual idea of how to do it or whether it can even be done and then dump it off on others.
I am assuming from the description that this is not a permanent installation. Based on that, have you talked to any local rental and staging companies? Since you would likely be relying on them for the equipment it might be worth getting their input. I do see a few specific challenges. For one, a 7'x20'
image is 2.85:1 cinemascope format, you won't find anything other than a true digital cinema
projector that directly supports that. That means that even though the
image area that you use may be 7'x20', to get the 20' width the
projector will likely be producing something more like a 11.25' to 15' high
image and the output of the
projector, and thus brightness of the
image, is based on that rather than the 7' height. This means your 5,000 lumens for the 7'x20'
image area becomes likely more like 8,000 to 11,000 lumens for the
projector and even that might be a
bit low for the application.
Second, you will almost have to somehow enclose the projection path from
projector to
screen. Without doing this you will have to overcome anywhere from around 1fc (twilight) to 1,000fc (
daylight) or even 10,000fc (direct sunlight) on the
screen. At 1,000fc, think of needing to provide millions of lumens rather than thousands.
Finally, trying to create a 20' wide
image with a 15'
throw is difficult enough but addressing that via a very short
throw lens then in turn most likely precludes the
projector being anywhere other than directly on the horizontal and vertical axis of the
screen.
It almost sounds like outdoor
LED screens might be a good solution, but I sort of doubt that the budget or electrical service for that is planned. Another option may be multiple projectors, maybe two widescreen format or three 4:3 format projectors, edge-blended. This would help
address the
throw issues and the
image brightness, but might not totally
address the off-axis projection and would still require light control for the projection path. Unless the only source is computer and the
edge blending can be done in
advance in software, you may have to look at a hardware based solution. Again, this is where talking to some local rental and staging companies that may have this equipment could help as far as the details and costs based on their inventories.