A couple of folks have clued me in to my mistake: I should have been trying to boot DQ0/1 instead of DU0/1. Now I'm getting disk activity followed by "%BOOT-F-Unable to locate BOOT file", which is better! I found 725/730 diagnostics on another tape image, so I'll try running those next.
Yay! -- Mark J. Blair, NF6X <n...@nf6x.net> http://www.nf6x.net/