It's an idea, but my thoughts are that people that ride trails when they are soft/too wet/muddy are the same people that won't buy the pass, aren't members of any mtb'ing association, do no trail work, and generally don't care about the damage they are causing.
People that MIGHT consider buying such a pass are already buying state park passes, off-road MTB passes, and maybe even HNF passes, in addition to MTB association dues...so I doubt they'd be willing to fork over whatever such a pass might be sold for.
Maybe more could just watch the weather a little more closely, get up earlier to ride while the trails are frozen/before they thaw, and bail if conditions warrant. Again, I like the "out of the box" thinking but in my opinion I don't think it would work for the reasons stated. We'll see what others say/think though!