Each imported icon needs to have an associated DISBTN.
Leave the normal icon as war3mapImported\BTNMyIcon.blp, but the associated DISBTN must be: ReplaceableTextures\CommandButtonsDisabled\DISBTNMyIcon.blp
(all DISBTNS must have ReplaceableTextures\CommandButtonsDisabled\DIS, the last bit being the same as the icon's name.)
edit: lol hivemind