-There are deals for fixed liquidity privately. Where the miners can sell their coins outside the open market at a premium or at a guaranteed price range for a set period of time
Care to provide any examples? I haven't heard of such "deals". Any proof that what you describe is in fact happening on any significant scale?
They are not. There are simply not large enough miners for them to have this kind of need. The OP likely got his "idea' from a recent article about coin brokers charging 5% for dealing in large amounts of bitcoin off exchanges.