Oh... Goody... Goody... Goody... My new jailbreak still works with the new diags partition (unless a new diags kernel somehow stops it -- but unlikely).
Just drop two files on the USB drive (one of them is a ENABLE_DIAGS that contains the payload). Reboot, select a menu item, and it runs. I am happy.
So, the question is -- should we publish this now, or should we stick with the "flash a custom diags with SSH"? I plan to add missing SSH files to the new 5.1.0 diags partition so that can be what we flash... [Done! My 5.1.0 diags has SSH. I put the mario passwd hash in the new mmcblk0p2_ssh.img file, and I successfully logged in with SSH from NUZX menu in 5.1.0 diags. All good... ]