Yes, DBC editing would be one way to do it, but it's actually possible with a small core edit too.
In MaNGOS:
File Player.cpp, find:
Code:
if(zone->flags & AREA_FLAG_SANCTUARY)
Replace with
Code:
if(zone->flags & AREA_FLAG_SANCTUARY || zone->ID == 876)
In ascent/arcemu:
File Player.cpp, find:
Code:
if(at && at->category == AREAC_SANCTUARY || at->AreaFlags & AREA_SANCTUARY)
Replace with:
Code:
if(at && at->category == AREAC_SANCTUARY || at->AreaFlags & AREA_SANCTUARY || at->ZoneId == 876)
Both these core edits would make GM island sanctuary. Just replace the id 876 with another zone id.
Imo, this is much easier than editing the DBC files. Also, its not illegal.
Of course you would have to compile your own core for this but if you don't do that already, you should really learn. It's not hard at all. Just download Visual studio 2008, start up and click "Build Solution". Thats how simple it is. Atleast in mangos, I dont know about ascent but it should be same.