If the CME and CBOE offer these futures, I assume most Futures brokers will offer it. The margin will vary from one broker to another due to their comfort levels with this contract.
This is why I sleep well with Interactive Brokers. They are risk-conscious and will be the last man standing in a financial crisis. If IB relaxes their risk standards, then it's time to switch to another broker.
Update: TD Ameritrade will NOT be offering BTC access on launch. They will be monitoring liquidity and "when the time is right" they will offer access.
So almost no firm will offer it. Those few that do will limit or prohibit shorting. Everyone is waiting to see how liquidity develops - that's a formula for success.