Link checker's URI scheme/protocol support comes from the libwww-perl library. 
At least in theory, all schemes supported by it should be also supported in the
link checker.  Now, regarding the cases from your example URL:

news://news.mozilla.org/netscape.public.mozilla.svg
This URI is not RFC1738 compliant.  There's no hostname part in news: URIs
according to the RFC.  Maybe it should be nntp://...?

news:netscape.public.mozilla.svg
This looks ok, but the link checker @ qa-dev.w3.org does not have a default NNTP
server configured.  Olivier, is there one @w3.org or somewhere we (c|sh)ould use
for qa-dev and v.w.o?

As far as I know, there's no standard for irc:// URIs.  That's probably why it
isn't supported in libwww-perl (and consequently nor in link checker).  I
suggest you contact the libwww-perl authors if you feel it should be supported.
http://www.iana.org/assignments/uri-schemes
http://www.w3.org/Addressing/draft-mirashi-url-irc-01.txt (expired 8 years ago)

Finally, news: and nntp: URIs are not too well supported in libwww-perl.  In
particular, they will only work sufficiently for link checker use for URIs that
identify an article; the implementation for URIs identifying a newsgroup is
pretty incomplete.  Yes, that means that even if the news->nntp "bug" would be
fixed and a default NNTP server configured for qa-dev and v.w.o, your URIs (both
of which identify a newsgroup, not an article) wouldn't be completely supported
by libwww-perl (nor link checker) at the moment.

(This made me remember that I have some patches that I should send to
libwww-perl upstream, those make the nntp support a bit better.)

I suggest making news: and nntp: explicitly unsupported.

For good, or for now (if/until LWP gets proper support for them)?

Well, v.w3.org should only report links as broken if there is no doubt that the 
link is broken, for most news: and nntp: links it cannot know that. We might 
support nntp://example.org which could check whether there is a DNS entry for 
example.org and maybe even whether example.org accepts connections to :nntp, 
but we should not do much more. As not supporting them at all is easier...

I just sent a patch to the LWP mailing list which makes the news:/nntp: support
in it complete enough for link checker use, excluding the case where the NNTP
server's hostname is not present in the URI.  And that case and how to work
around it has been documented in the link checker documentation for some time now.

If my patch is accepted to LWP, I'm not in favour of dropping the feature
altogether.  We could possibly add a special message and short-circuit without
even trying for the no-server-in-URI case.  Or do that only if a default NNTP
server is not defined, but the problem with this approach is that Net::NNTP's
default server resolution is somewhat hairy and not exposed through the API.

Keeping the feature requires no changes to checklink, removing it does :)

My news/nntp patch is included in LWP >= 5.804, so as far as I'm concerned, this
bug can be resolved by upgrading LWP on v.w.o.  Reassigning to Olivier for that.
 Note: I haven't personally tested libwww-perl >= 5.804.

Currently, all that's available on any debian distrib is libwww-perl 5.803-4.
http://packages.debian.org/libwww-perl 
For the sake of maintenance of the validator servers, I'd like to wait and see whether/when 5.804+ gets 
into debian.