I have thought about this some more and looked more carefully at RFC 3798, and I don't think RFC 3798 MDNs should be considered bounces. The first paragraph of the Abstract of RFC 3798 says:
This memo defines a MIME content-type that may be used by a mail user agent (MUA) or electronic mail gateway to report the disposition of a message after it has been successfully delivered to a recipient. This content-type is intended to be machine-processable. Additional message headers are also defined to permit Message Disposition Notifications (MDNs) to be requested by the sender of a message. The purpose is to extend Internet Mail to support functionality often found in other messaging systems, such as X.400 and the proprietary "LAN-based" systems, and often referred to as "read receipts," "acknowledgements", or "receipt notifications." The intention is to do this while respecting privacy concerns, which have often been expressed when such functions have been discussed in the past.
The key point is that the purpose of an MDN is for an MUA to report the disposition of a message after it has been successfully delivered to a recipient. Thus, it's use by the Dovecot LDA to report non-delivery due to over quota or other reasons seems incorrect, and Dovecot should be sending a DSN with a 'failed' action for the recipient rather than an MDN with a 'deleted' disposition which is intended to report that the message was deleted by the recipient after successful delivery and which should not in general be interpreted as a bounce.
Thus, I think Dovecot's behavior is incorrect, and the best we can do in Mailman is to recognize the "Your message to email@example.com was automatically rejected:" line in the simple text matcher as a work around.