Its only regulated that that most who partake are FDIC insured institutions (banks).  IANAL, but I don't see why two parties can't enter into a private contract in which the holder of some asset agrees to give the assets to another for a fee and a promise to redeem that initial asset at some later contract defined period.  Governments are more interested in your role in MOVING assets (money) around and less interested in if you pay out fees for holding it... but IANAL.

btw... how does your idea involve paying out interest on btc holding?