Ineeed, I ended up simply doing a local hack and editing the PLIST file,
but it would be good to see a solution incorporated into pkgsrc itself.
I don't claim that the solution that I provided previously is the
"right" one, but it works and I haven't seen any proposals for
alternative solutions.
With regards to the directory mismatch issue, an alternative solution
would be to alter how MACHINE_GNU_PLATFORM is generated so that it is
more consistent with autoconf's config.guess. That seems like the more
correct approach, but it would impact more than just Octave.