[johnnym]

Small update:

Some other output stands out, in particular the usage notes indicate that something is wrong with the parameters being supplied to those utilities in the rc files. Also "unexpected !=" means there is a syntax error, possibly caused by an unset variable.
You can copy and paste one line at a time from the rc file to see where the problem first occurs.

I'll have a look into that, thanks for the pointers.

I did examine the multi-user boot process in more detail and enabled and disabled various things in the rc script, but I couldn't find the exact point where things start to bomb. To me it looks like something important changes when the machine leaves single user mode or enters multi-user mode directly, but I can't figure out what. 

In the meantime I also tried an on-disk installation using a 6.9 sgi RAMdisk kernel for IP28 (looks like I indeed can't build a new one without a release IIUIC) and a 7.2 octeon base72.tgz disguised as base69.tgz using my 7.2 sgi IP28 kernel as /bsd. This worked well, except for the bootloader installation, but thanks to the excellent OpenBSD manpages (sgivol(8)) I could do that manually in a shell after the installation had finished.
And this shows the same picture: Things look good in single user mode, but fall apart in multi-user mode.

But judging from my on-disk install experience and with all the other machines (see above) working correctly, this also means that on-disk installs are already possible now with:

• 6.9 sgi RAMdisk kernels,
  
• correctly named 7.2 octeon sets,
  
• 7.2 sgi kernels from GitHub
  

...and with some manual work afterwards. Maybe I should create a howto on GitHub.