Hi,
You can't use the getShippingAddress to inject a custom shipping address as a normal shipping address.
The getShippingAddress allows you to completely replace the shipping address interfaces to only display your text.
So for example, you can store your address data in a custom field of the table "order", and in the getShippingAddress function you can just return the HTML of your address that you want to appear instead of the normal shipping address interface.
Or you could implement the onBeforeOrderCreate function and replace the shipping address fields in $order by your own data.
That way, the address would be replaced when the order is created and it would act like a normal shipping address, that you can edit in the order backend interface, etc.