Thanks to Dana and John for the comments, which I think were helpful.
Here's another cut at the new paragraph:
OLD VERSION:
NOTE: In certain transport domains, the RFC 822 restriction that limits
body-parts to printable ASCII characters may have been removed. Such
removal should be construed as locally extending the definition of
body-part to include octets outside of the ASCII range.
NEW VERSION:
NOTE: In certain transport domains, RFC 822 restrictions such as the
one that limits body-parts to printable ASCII characters may not be in
force. (That is, the transport domains may resemble standard Internet
mail transport, but without certain restrictions.) The relaxation of
these restrictions should be construed as locally extending the
definition of body-part, for example to include octets outside of the
ASCII range, as long as these extensions are supported by the transport
and adequately documented in the Content-Transfer-Encoding header field.