[Arnavion]

Of course, despite working on this for a week I only now discovered this... dm_linear is an easier way than mdadm to concatenate the loopback devices into a single device. Setting up the table input to `dmsetup create`'s stdin is more complicated than just `mdadm --build ... /dev/loop1{0000..9999}`, but it's all scripted anyway so it doesn't matter. And `mdadm --stop` blocks for multiple minutes for some unexplained reason, whereas `dmcreate remove` is almost instantaneous.

One caveat is that my 1MB (actually 999936B) block devices have 1953 sectors (999936B / 512B) but mdadm had silently only used 1920 sectors from each. In my first attempt at replacing mdadm with dm_linear I used 1953 as the number of sectors, which led to garbage when decrypted with dm_crypt. I discovered mdadm's behavior by inspecting the first two loopback devices and the RAID device in xxd. Using 1920 as the number of sectors fixed that, though I'll probably just nuke the LUKS partition and rebuild it on top of dm_linear with 1953 sectors each.