Ah! I had set use-nvramrc? back to false in one of my attempts at 
resetting things.
So now, having having got rid of everything in nvramrc except
devalias bsd /pci@f2000000/mac-io@17/ata-4@1f000/@0:9,ofwboot.xcf
I can type
boot bsd
and it works.

I'm still confused about
* why nvalias stores as a devalias
* why nvstore STILL tells me nvramrc is invalid. I was following the 
INSTALL doc, which gives this as the storing aliases sequence:

0 > nvalias bsd /pci@f2000000/mac-io@17/ata-4@1f000/@0:9,ofwboot.xcf
0 > nvstore
0 > setenv use-nvramrc? true
0 > reset-all

Because there is a difference between the updated OF 3 and non-updated 
OF 3?

Um, having been helpful with this problem, could anyone tell me how to 
get past the other one that's really annoying me at the moment, which 
is that using pkg_add fails on any package:

tar: invalid header, starting valid header search.
gzip: (stdin): unexpected end of file
tar: end of archive volume1 reached
tar: unexpected EOF on archive file
pkg_add: [...failed...]

Redownloading doesn't help, and they open on OS X, so I don't think 
it's a problem with the package files. I have the same error trying 
with the packages located on a flashdisk or other partition as locally. 
I can't try adding through FTP as I have no airport card or 
BSD-compatible modem at the moment.

Another thing: who am I meant to address messages to on this list? I've 
been sending them always to port-macppc@netbsd.org, but other people's 
(at least replies) seem to be addressed to individuals with a Cc to the 
list address.