Accessing aliased object's properties only works if the property has been declared in C++

Define IconButton component:

import QtQuick 2.0

MouseArea {
    property alias icon: icon
    Image {
        id: icon
        property string custom
    }
}

You can access aliased object's properties when they have been declared in C++:

IconButton {
   icon.source: "icon.png"
}

But not when the object's property has been declared in QML.

Executing

IconButton {
   icon.custom: "Value"
}

results in error "Cannot assign to non-existent property "custom".