I never tried it, but it seems like that would be a problem. The other issue will be having enough distance from the
scrim, and the correct
lens, to get the
image size you need.
I've not done rear projection on a
scrim, that said I have some experience with rearprojection on a
screen. The
hotspot can there be a problem, which can be handled by putting the
projector low behind the black curtain (how do you
call that?) under the
screen so the public can't see the
projector.
Distance can be handled in two ways. With a short
throw beamer but they tend to be low in
lumen.
So when I have to turn our gym into an
auditorium for the graduations I use a mirror. The beamer stands below the
screen, behind that curtain and points away from the
screen into a mirror which puts the picture on the
screen. It nearly halfs the needed distance, although still more then a short
throw would need.
Two pictures, from our graduations in 2019 and 2022. The
hotspot in the latter is part of the picture ;-)