Thats odd... i could swear that i had some item based abilities with an icon.
I cant remember what abilities they were but all i can think about is unchecking the "item ability" checkbox.
In any case, you can create a dummy ability (an ability that has no effect) to show the tooltips and icons.
You can create a dummy ability based of attribute bonus and set all data to 0.
You have to make sure that every unit that has one of the two also has the other.
If you want to have it a hero ability, you need 2 triggers that learn and increase the real ability whenever the dummy is leveled up.