UMA DOES HAVE their own clubs and events
P-51B - Read the UMA website closely. You will see that they are doing EXACTLY what you propose.
Here is the program as I interpret it-
Charter your club as UMA instead of AMA.
Your club CAN accept either UMA or AMA members and both can fly fully insured at the same location.
UMA insures the site owner and club officers.
Whichever organization the individual has chosen will cover that individual's liability.
If you sanction your event as a UMA event then AMA members can also fly with full coverage.
The only downside I see is AMA not accepting UMA for their events. As I said before it will probably take a move on the part of the EC to add more restrictions for the UMA to really get traction. Should this happen then I could see the turbine and IMAA types organizing under UMA for their events. Most folks in that part of the hobby would probably spend the $35 extra per year to belong to both if it meant less restriction on the big stuff.