Lutsen Mountain did at one time allow unicyclists but has since banned all cycling on their trails, citing erosion as the primary reason.
It was Mammoth Mountain in California in, I think, 1987. I know I struggled getting down on the loose gravel of the track on my Pashley 24 which I had converted to a 26" with an offroad tyre.
I got lots of postive feedback when I did it I know that. 
Roger
Thanks Roger. That’s got to be some historical early MUni riding! If you still have a copy of that magazine/cover it would be great if you could scan it and post it here and/or send me a copy. 
I would love to find the copy I had. I think I gave it my mother years ago and hoped it was in her loft… it was not. I have also talked to MBUK about it and their records that far back are not complete but have promised if they find it to send me a copy.
Lets hope one turns up… not as if it is a really exciting picture, just me on top of a mountain doing a still stand with my arm out.
Roger
I am sure I have seen this photo, years ago. So it’s possible there’s one somewhere online. Might be worth a bit of searching.
Kris
I’m glad this thread came up. I’ve wanted to go ride muni at Mammoth Mountain, but it never crossed my mind that it might be a problem. In my head I just thought “Oh it’s a small item, it won’t be an issue”
Thanks for lookin out for the community Kris!