It seems that IPv6 and TLS support are both delivered as patches.  From
  the install doc

	  Note: support for TLS (encrypted SMTP sessions and certificate
	  based authentication) and for IP version 6 is still separate from
	  Postfix but is expected to be merged soon.

Yes, but the patch that pkgsrc uses has both IPv6 and TLS in it:

PATCHFILES+=		tls+ipv6-1.24-pf-2.0.20.patch.gz
PATCH_SITES+=		ftp://ftp.stack.nl/pub/postfix/tls+ipv6/1.24/

You pointed out:

  http://www.aet.tu-cottbus.de/personen/jaenicke/postfix_tls/

This is TLS only, with no v6.

  http://www.ipnet6.org/postfix/ipv6.html

This is TLS and v6, and is what pkgsrc is using (from a mirror site
and now one patch rev behind).

  That's a path I'd rather not travel for the base when it can be delivered
  with pkgsrc.  Please correct me if I've missed something here, which is
  quite likely.

I think it's deficient of our in-tree postfix to be missing v6 and TLS
- sendmail has them out of the box.  But given that postfix is finally
going to integrate these patches, it doesn't sound like a good use of
time to integrate the patch into the base.