If you really need long-term heat resistance to 300 deg F, you will find only a limited number of elastomers will serve. If the rubber only sees the heat and pressure (from air or inert gas) and you don't need oil resistance, EPDM might do the job OK. If the bladder is going to inflate/deflate a lot, which means a lot of flexing, then you need flex resistance as well. Silicone will take the heat, but maybe not the flexing. An engineer in the rubber business will want to know a lot more details about the function. And by the way, if this is not going to lead to orders for a serious number of bladders, it'll be hard to get anyone interested in the requisite development work. Good luck.
R J Del Vecchio
Technical Consulting Services
(consultant in Applied Rubber Technology)