Creepage Distance in Altium Designer
You can define several footprints for the same opto IC. Define a HV-Footprint which includes the slot in the footprint. 

Then modify the DRC and exclude the opto IC from the clearance 8mm. Due to the slot in the footprint this should be OK, as you cannot forget it.

I would do it this way:


In the schematic: add a parameter name: "ClassName" and value: "exclude_8mm_clearance" to every IC you want to exclude. This will add this devices to the component class "exclude_8mm_clearance"
In the PCB add "NOT InComponentClass('exclude_8mm_clearance')" to your design rule