In Island defense, the golding ability is a copy of the human 'repair' ability. The gold pile is not invincible, but has a ton of armour and health.
Copy the repair ability and change the 'targets allowed' to enemy+structure, then make your gold/wood pile for an enemy team. The reason that mine gold has to be autocast in Island defense is so that the builders wont attack the gold, and will repair it instead.
Then you must use a trigger to add gold, check if the unit is casting/channeling the ability, and add x gold to player every y seconds.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.