I didn't travel for the 1990 game. I believe the sign was referenced in Sports Illustrated, though.
@Highlander attended that game. Did you see the sign, dude?
I DID travel to the 1992 game with Burrito and a series of other knuckleheads. There was at least one sign (in my memory there was more than one, but I know for sure there was one - we stopped, ran back and took it down, I think) travelling into Lincoln that said "Sal is dead, go big red". It wasn't big. It was cardboard (either on a wooden post or attached to an existing highway marker).
So I can personally confirm at least one sign in 1992.
The husker fans in that thread are amazing, though: "Nooooo, it wasn't a banner! It was just a bunch of signs and a chant!" At least they're owning it, I guess?