I'm OK with non-BMW riders. My only concern would be inexperienced sportbikers riding beyond their limits.
In fact my friend, who suggested Richland Center, is interested too. Saturday the 19th is her BDay, she is a member of the Mississippi River BMW Club but rides a HD Sportster

(not the loud pipes!) She's been all over the US on that thing too, East Coast, Sturgis at least twice, Deals Gap, etc.
So, for me the count would be 3 if OK. (Me on the R1150R, my wife on her V-Star, and our friend on her Sportster)
Also: in an earlier post I mentioned staying over at Prairie, may not be a good idea, as Summer rates will kick in. PDC is expensive in summer $85 and up even at Super 8. Maybe you know some lower cost places or towns.