I disagree. RFC822 says that the body of an internet message is plain
text in the US-ASCII charset. That doesn't stop us from sticking stuff
in internet messages that satisfies the letter of RFC822 and applying
a different interpretation to it -- MIME.
Suppose we leave the normative part of the finger protocol alone,
but we extend our usage of it so that we interpret what comes back
as a text/plain body part, unless by some means (e.g. an HTML link)
we can determine that it should be some other text/* content type.
Note that what comes back still satisfies the constraints of the
finger protocol -- we just interpret it a little differently.
Normal finger clients see some SGML <tags> that they didn't bargain
for, but I don't see much harm in that.
Now folks can't stick image/gif data in their .plan file, but they
can put pointers to them.