If the holes are plated you can tie the mounting holes together using the add connection tool. After connecting the mounting holes switch back to the select object tool and right click on the ratsnets line between the pins and select properties. On the nets tab set the appropriate spacing.
If the mounting holes are non plated you can place a small plated hole in the center of the larger mounting hole and use the above mentioned method to create the copper pull back. This will result in a drc warning but you will get the copper setback you want.
The only other way to get the copper setback you want without using the provided work around is to manually edit the filled polygon to avoid the mounting holes.
Please contact customer support if you have any questions.