MDNs and DSNs are rather unusual MIME multipart messages. The fields
(N.B. they are described as fields in those RFCs) are in fact defined
using RFC 822 syntax (both RFCs predate RFC 2822):
no, they are defined using ABNF notation, which is not the same thing.
As author of 3 of the 4 DSN RFCs I can confidently state that
the structuring conventions of RFC 822 header fields were not intended
to apply to DSNs fields. And as author of RFC 2047 I can confidently
state that encoded-words are not intended to be used or interpreted in
message bodies (except for message bodies that by their definition
contain 822 messages or message headers)
Keith