Delivery Options if Out of Stock - option to sub / customise

At present for out of stock delivery options, you can choose between single delivery or multiple deliveries. It would be great to have a third option 'to sub products' or the ability to customise another option here. We sell food products and when we experience out of stock items we sub to a similar product however there is no option to articulate this nor area of the portal to clearly communicate this at checkout stage