To answer my own question it seems that MTD (Memory Technology Devices) are not quite normal block devices, and jffs is aware of that. Jffs will not run on top of anything but an MTD, and you can not simply DD a jfs image on and off. It does work under some circumstances I do not understand, but not in the common case.

You can DD off of an MDT (from /dev/mtdblockN), and then mount that image (via a loopback device AND a weird block to MTD converter) but you can't, as far as I can tell, DD onto an MTD. I expect that a UBIFS filesystem will work the same way.

Can anyone explain why this is difficult? It looks like /dev/mtdblock1 is the kernel image, and /dev/mtdblock2 is the root filesystem. I would naively assume I could dd both to images on my bootable USB stick, boot a fresh plug from same USB, then dd those images back onto the new plug's flash?