I eventually managed to almost properly mount an Ultrix file system on 
NetBSD: I created a large 'a' partition just at the beginning of the 
disk. It mounted okay, the ls was fine, and I was able to start untarring 
the 94 MB base.tar. However, after a couple of dozens of files, things 
went wrong with a bunch of messages like

/tmp/ultrix/tar: read error on /tmp/ultrix/base.tar: Invalid argument

I haven't followed that up any further, but given the amount of data 
transferred successfully, it could well be that reading the Ultrix file 
failed as it came to the first backup superblock on the partition.

I then turned back to my exabyte and managed. Here's the recipe (thanks 
to Donn Cave): Under Ultrix, copy the tar file onto the Exabyte via

dd if=base.tar of=/dev/rmt1l obs=1024 conv=sync

The tape so created could be read successfully using tar on NetBSD.

Helge