That would be similar to the auto-generated comment after someone votes to close a question as duplicate.
Imagine a question is off-topic and gets closed. The users that closed the question selected an alternative website while closing.
When the question is closed there is no hint that there might be a very similar site where (s)he could ask the question like superuser.com for example.
How about leaving an auto-generated comment like
This question would be a better fit for superuser.com.
if the users that closed the question selected a unique alternative website?