I'm not sure this is required since the AMI's should already have the blocking baked in. https://github.com/riptano/ComboAMI/blob/2.5/presetup/setup.md#baking-script-ebs-backed

If it is, could you add all the way to sdy: ephemeral23? It's my understanding that you can overdo the block device mappings, but it won't matter unless you have those ephemeral devices. A simple test of a m1.xlarge with the extended block mappings should suffice.

If that works, then you can edit the caveat above regarding the block device mapping.