I've been to Weeki Wachee Preserve a couple of times, and it could be a great place to build some trails. Its got plenty of places to build some very scenic green trails, and it's definitely got some mounds and rough areas that could be nice blue/black trails.
All the Starkey Park Trails were recently built on SWFWMD property, and I believe Balm Boyette is as well, so it is possible. I met Rudy Miller recently, who was one of the originals that went to battle to get those trails built. As I recall, he said it took a few years and getting the Starkey Family's political power behind him to get it approved ... but, it is possible.
With biking already being allowed at the Weeki Wachee Preserve, maybe it would be an easier sell? I don't know where to start, but if there is a group getting together to try to start something there, keep us posted. I'd help with whatever I have to offer.