See, all the info I keep finding is Mt. Bike trip reports talking about how the trail was made from the beginning as a Mt. Bike Trail. So I have my doubts. The only hopeful thing I've found is this map here:I've actually mountain biked out there, years ago. I don't remember that it was open to motorized vehicles, but that doesn't mean it anything.
I did think that there is some potential for big dual sport rides in the area...