Hi Martin,
I may haved missed some details in your original post, can you please describe in more details how you did boot/run the installer?
beside the general method I've described, the interesting part for our matter should be how I got my installation system up and running. In brief, I used the GENERIC kernel and the provided rootfs.tgz by NetBSD/sparc 6.1, which I simply extracted using tar xzf. Yep, that was my mistake, I missed the 'p' option for tar. I should have read the branded sparc/INSTALL guide instead of re-inventing the wheel by testing various papers on the web.