You might be able to do the
effect with two towers on either side of the
stage, with your
release points only at the top corners of the
drop. However, getting the top to pull taut to anything resembling a straight
line will be difficult. If a somewhat drooping
swag at the top of the
drop is acceptable, then you only need to come up with towers suitable to your needs and budget. I'd personally tend towards
truss stood on end, with corner blocks and short sections forming a stable
base of outriggers. If the duration between the start of the show and the
drop effect is short, a couple actors/crew holding some pipe vertically might suffice, although the
drop might move when the holders fidget.
Otherwise, you're into
truss spans and crank-up towers, which might not be within your budget or transportation ability.
I'm assuming that you're dealing with all indoors venues - outdoors would require wind loading to be considered, and things like free-standing drops get much more bothersome to deal with.