Okay, I understand.
It’s not possible to access the owners mapping, this is by design and it won’t change.
My suggestion to conserve gas would be to first update to version 4.0 Beta, which has improved the gas efficiency of ERC721. (See announcement for details.)
However, you have to use _mint in the for loop. Take a look at the gas costs after upgrading to 4.0 Beta and see if you still consider that it needs optimization.