Just to make sure I follow, there is no evidence of this being done at all or just no evidence that it was definitely Husker fans?
Not sure. I've never seen this sign, or a picture of this sign, or news articles about this sign, and I've searched and searched and searched for it. The only place I've ever seen it discussed as fact was on Buff message boards, and one person here (forget who) who said they saw it.
Regardless of whether there's evidence of it actually happening, there's ZERO evidence about who did it. Heck, it could have been KSU fans trying to stir up trouble. Pinning this on the Husker fan base is pretty flimsy. It wouldn't last a second in any kind of court.