I tested following up by NNTP from Gnus, and ended up giving it up. Here's my running commentary: 

I'm testing following-up from Gnus. I'm able to read via NNTP, though
the only visible group is slc.all. I don't *expect* posting to work,
because the followup buffer is missing a bunch of headers I would
generally consider essential (like Newsgroup, References). But we'll
see!

First try to send: Message-mode couldn't figure out how to send the
message. It does automatically construct a References header now, so
that's helpful.  I'm adding the Newsgroup header to try
again. Following-up to groups not on your primary NNTP server is not
always easy in Gnus, so we'll see when I try to send. 

Okay, that posting buffer didn't know it was responding to a newsgroup,
and changing the headers didn't fix it? Creating a news posting buffer
from scratch and copying over headers.

Nope.

Oh! The header is "Newsgroups". Guess it's been a while since I did much
NNTP.  Okay, now it tries to send via my default news server, which is
not good... making it try harder.

```
(setq gnus-post-method (cons gnus-select-method
                             gnus-secondary-select-methods))
C-u 0 C-c C-c 
```

Nope, still no good.