I'm guessing it depends on the exchange you're using. Kraken issued the following
statement:
(Note that they refer to BCC as BCH and BTC as XBT)
Long XBT margin positions held across the fork will result in a BCH credit
If you hold a 10 XBT long position across the fork, it will carry over and can be closed at any time in the usual way. However, you will also receive a 10 BCH credit to your balance corresponding to the margin position.
example 1: margin buy (long) 10 XBT/USD = +10 BCH (long xbt, short usd)
example 2: margin sell (short) 10 ETH/XBT = +10 BCH (short eth, long xbt)
Short XBT margin positions held across the fork will result in a BCH debit
If you hold a 10 XBT short position across the fork, it will carry over and can be closed at any time in the usual way. However, please note that you will also be responsible for a 10 BCH debit (negative balance) on your account that must be corrected by either buying or depositing 10 BCH. Why the negative balance? Kraken sources the XBT to finance margin trading from individual lenders and we are preserving the value for these lenders that may lie in the BCH forked from their funds. This means that traders who borrow XBT across the fork to short must also pay back the BCH that forks from this XBT. Any account with a negative BCH balance will be placed on withdrawal lock until the balance is corrected.
example 1: margin sell (short) 10 XBT/USD = -10 BCH (short xbt, long usd)
example 2: margin buy (long) 10 ETH/XBT = -10 BCH (long eth, short xbt)
Margin traders should be very cautious across the fork
Margin traders are advised to be very cautious across the fork, by either reducing their position sizes or closing out entirely before the fork. In addition to the special provisions described above, margin traders should plan for the possibility of extreme volatility and unfavorable forced liquidations surrounding the fork. As always, margin traders are strongly encouraged to set stops rather than rely on perfect accessibility to the exchange.
I suspect other exchanges may have different rules about it, though. Sounds like you need to tread carefully whichever exchange you're operating on. If in doubt, open a support ticket with them or email them directly, as that's likely to get you a more accurate answer than this forum will.
