I talked with the DNR enforcement dept again yesterday. They've been very helpful but, at the same time, somewhat slow moving. There's another provision in MN law that says we can get a permit from the DNR that exempts out of state bikes from needing a MN permit at our event. That's what I applied for yesterday. I was told it should go through but I'm not going to count on it until I see it.
We will have the DNR tags for sale at the event. They are $32 total and are good for 3 years. And, as Layton said, ALL MN RESIDENTS are required to have one to ride any public land in MN, no exceptions.
What Layton described above is accurate about out of state requirements. There are some fuzzy issues, though. For example, if your state requires an offroad sticker to ride on trails and you don't have it, you'd need to buy the MN sticker to ride here. But if your state doesn't require a sticker at all to ride on public lands, MN gets confused. It seems you should be legal to ride here in that case since you're legal in your home state but it seems to get enforced that you need to have the sticker.
They're trying to iron all of this out and make it more straight forward. In the meantime, the permit I applied for above would be a great answer for our trailride. That permit would, of course, apply only to our ride.