When an ILL item is checked out to a patron and must be returned, we should be able to print a slip with useful information. The information and the model could be different from the ordinary "check out" slip. And even a library not using "check out" slip for ordinary check out could have the utility of a "ILL check out" slip.
I am sure you are getting tired of this, but just to give an additional idea. We print the slip when the item is "received" in the library, before checkout. It goes in book and contains a scannable barcode for the item. This is then moved to the circulation desk. What kidn of information woudl you want to print on checkout?