Panasonic has an ultra short
throw lens for their larger projectors, specifically designed for this application.
Projector actually points toward the upstage wall and reflects the light back toward the
cyc. Unless it's a pretty small
screen size, you could be talking about a $25k-100k investment though depending on the size of the projection surface, type of projection surface material, how wide the viewing angles need to be, and any distribution/
media server/playback requirements.
I wouldn't recommend using the existing
cyc, especially if has seams. Even if it's seamless you'll have to spend more money on a brighter
projector to cut through a
muslin cyc than you would using a dedicated RP
screen. Some of the RP surfaces like you can get from Rose Brand actually have a high rear projection
gain so you do not need to pay quite as much for a high-output
projector. The caveat is that the high
gain screens tend to be worse at rejecting ambient light from the front.