Not sure if it would work exactly like a DSN, but something worth testing would be a routing rule that sends a response to a message sent to the alias that at least sounds like a DSN. 

I think I understand the design problems here.

(1) The message is relayed from user1 to user2.   User2 is overquota.   SmarterMail could send a message to the sender "user2 has a full mailbox."   The sender will not understand the message because he did not send a message to user2.   Do you even want the user2 address exposed to the sender?

(2) Suppose that we solve that problem and "overquota" messages are sent on "from" user1.

However, the alias is a list of multiple addresses, and only one mailbox is overquota.   Do you tell the sender that the message was delivered (which is partially true) or not delivered (which is also partly true)?

Given those complexities, silence seems like the least bad result.