There are many ETFs that have a standing swap with their inverse fund to offset one another.
The real problem with a move of said magnitude is that the mechanism which ties the ETFs market price to the net asset value of the underlying (NAV) is likely to break. This mechanism known as creation redemption may be halted. Moreover, if the ETF were to halt prior to the index, it becomes impossible for the abiteurs to tie price to NAV. Some ETFs run a consistent premium and discount to NAV for this reason. Deutsch Bank has issues many such products.