Don't totally rule out the possibility that it will be a little bit cold at night. Here in FL we're talking end of winter, hit or miss at to when the cold fronts stop coming in for the year. Can't hurt to bring something warm, though if you're coming from further north it will probably just feel like perfect weather to you :-) (more worried about the heat arriving too soon... good choice that they did it EARLY March)
And yes, shortage of shade in the camping areas, at least in at the Phish BC event the campsites were not near any trees. Rain... likely to happen at SOME point during the event, but not a big threat unless your tent leaks (now THAT sucks!). And Anabolic is right about the bug spray.
Early March is a good time. There is a very good chance the weather will be beautiful.
Any other advice I can think of is just the same stuff you need to know at every jam festival, like bring your own toilet paper, etc. My current favorite link is: http://www.oldmusicproject.com/Campers_Advice.html
About the cops... not sure about that not allowed on the reservation thing... I did see cops at Phish BC, but they were all very obvious locals, no crackdown forces from outside. The security WAS extremely lax... all kinds of goodies were shared with little to no paranoia... all they wanted to do was keep violence and destructive shit from happening. BUT, don't be overly complacent. I was stopped by a cop who I believe was a Seminole, for no good reason, and had my pockets searched.
As for the grounds in general, I am quite certain that the people running the actual event will hold much more sway than the Seminoles who run the reservation, in terms of security and all that stuff. I would expect a laid back scene, but to be honest, my take is that you really just have to feel out what the security is like at each individual event you go to.
One thing though, the Phish crowd were terrible with leaving their trash (along with piles of shit and used toilet paper in the woods) so there might be a bit more of a crackdown this time with keeping the place clean
hope that informs! :-)