Still trying...
Thought I'd start with the obvious, so I tried "run boot_maint", but that also appears to rely on the RAM image and wouldn't boot (same error as boot_user).
I tried "run boot_maintsd", which looks for an image (I assume) on the microSD card. I didn't have one inserted, so needed to hit the reset button to get out of that mode. That may be a good method to investigate if I can find out what the bootable image should look like.
Next, I tried "mtest" which is described in the monitor help file as "simple RAM test".
I got the following:
Code:
EdgeJr>> mtest
Pattern 00000000 Writing... Reading...
Mem error @ 0x00CEF81C: found 0000000A, expected 002FBE07
Mem error @ 0x00CEF820: found 00000000, expected 002FBE08
Mem error @ 0x00CEF824: found 00F15DFC, expected 002FBE09
Mem error @ 0x00CEF828: found 00F2BDE6, expected 002FBE0A
Mem error @ 0x00CEF82C: found 00CEF828, expected 002FBE0B
Mem error @ 0x00CEF830: found 00CEF828, expected 002FBE0C
Mem error @ 0x00CEF834: found 002FBE0C, expected 002FBE0D
Pattern FFFFFFFF Writing...
It never came back after that (I gave it about 15 minutes).