I think I found my answer
He starts with a RAID 1, then converts to a 2 disk RAID 5 and expands to 3 disks. The raid conversion is done directly with mdadm after converting to Compatibility Volumes in EVMS. This works because RAID 5 on 2 disks is a mirror with RAID 5 metadata. After that, EVMS is used to convert from the Compatibility Volume again and expand the array. Someone replied to this article stating it worked with his real drives and real data.
Definitely not the outcome I expected, but this certainly makes me happy.
This took me some thinking to get my head around why a RAID5 of 2 disks would really be a mirror. Here's why:
First, familiarize yourself with RAID 5
Now, if we have 2 disks, one disk will be data for a block and parity for the next, and vice versa. Since we only have 1 data block, parity pN will be data dN XOR 0 = dN=pN. It is a mirror!
Disk 1....Disk 2
The only caveat is that this requires madm knows how to calculate the parity when only 1 data block exists, which apparently it already does, so we're ok. Definitely worth experimenting with LOOP devices just to be sure madm hasn't changed.