Woody can now make one.
Be aware that RAD makes these hubs from Woody's initial suggestions and design. Under no circumstances would I ever buy them from RAD as they modified the bearing design from Woodys' specs and there have been 4 complete failures that I know of, we evaded failure by some diligent pre flight inspection and after much investigation Woody discovered RADs' un documented and badly executed alterations to his design.
Woody now gets the hubs from RAD and re specs them back to his original and much more robust design. If you want to see them in the flesh, PM me.
Hmm just noticed you wanted a 21", ours are standard size but I see no reason why Woody couldn't lace a 21" rim onto the hub. Then the only difficulty might be clearances.