dd-format images fail to deploy

I'm using MAAS from the experimental PPA, version is 1.7.0~rc3+bzr3299-0ubuntu1~trusty1 (just upgraded this morning).

I'm trying to deploy a ddtgz format image, but the image is never sent to the node, which then appears as "failed deployment" in the MAAS UI.

The image is based on an OEM version of Ubuntu (though as you'll see below, I think this would happen with any other dd-format image). To create my image, I installed this in a VM, then shut the VM down, connected the qcow image and did "dd if=/dev/nbd0 of=myimage.dd". Finally, I tarred the dd file and uploaded to MAAS like this:

time maas roadmr boot-resources create name=my-image-dd title='roadmr dd image' architecture=amd64/generic content@=/home/roadmr/download/myimage.dd.tar.gz filetype=ddtgz

maas boot-resource read shows this:

roadmr@thinkpad:~$ maas roadmr boot-resource read 22
{
    "name": "my-image-dd",
    "title": "roadmr dd image",
    "sets": {
        "20141118": {
            "files": {
                "root-dd": {
                    "filename": "root-dd",
                    "filetype": "root-dd",
                    "sha256": "e2fc5a1259df5b658d714aee29cfe7580a31a3ed9be69cd8a69f3cdd565f8baa",
                    "complete": true,
                    "size": 1890612863
                }
            },
            "label": "uploaded",
            "version": "20141118",
            "complete": true,
            "size": 1890612863
        }
    },
    "architecture": "amd64/generic",
    "subarches": "generic",
    "type": "Uploaded",
    "id": 22,
    "resource_uri": "/MAAS/api/1.0/boot-resources/22/"
}

then in the UI, I "acquire and start" the node, which boots the installer, then barks something at me, then reboots and gets stuck at the "boot:" prompt. On the UI I see "Failed deployment" and this is in the install log:

--2014-11-18 21:47:33-- http://10.10.10.1/MAAS/static/images/custom/amd64/generic/my-image-dd/uploaded/root-tgz
Connecting to 10.10.10.1:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2014-11-18 21:47:33 ERROR 404: Not Found.

gzip: stdin: unexpected end of file
tar: Child returned status 1
tar: Error is not recoverable: exiting now
Unexpected error while running command.
Command: ['sh', '-cf', 'wget "$1" --progress=dot:mega -O - |tar -C "$2" --xattrs --xattrs-include=* -Sxpzf - --numeric-owner', '--', 'http://10.10.10.1/MAAS/static/images/custom/amd64/generic/my-image-dd/uploaded/root-tgz', '/tmp/tmp1LPsHt/target']
Exit code: 2
Reason: -
Stdout: ''
Stderr: ''
Installation failed with exception: Unexpected error while running command.
Command: ['curtin', 'extract']
Exit code: 3
Reason: -
Stdout: '--2014-11-18 21:47:33-- http://10.10.10.1/MAAS/static/images/custom/amd64/generic/my-image-dd/uploaded/root-tgz\nConnecting to 10.10.10.1:80... connected.\nHTTP request sent, awaiting response... 404 Not Found\n2014-11-18 21:47:33 ERROR 404: Not Found.\n\n\ngzip: stdin: unexpected end of file\ntar: Child returned status 1\ntar: Error is not recoverable: exiting now\nUnexpected error while running command.\nCommand: [\'sh\', \'-cf\', \'wget "$1" --progress=dot:mega -O - |tar -C "$2" --xattrs --xattrs-include=* -Sxpzf - --numeric-owner\', \'--\', \'http://10.10.10.1/MAAS/static/images/custom/amd64/generic/my-image-dd/uploaded/root-tgz\', \'/tmp/tmp1LPsHt/target\']\nExit code: 2\nReason: -\nStdout: \'\'\nStderr: \'\'\n'
Stderr: ''

So curtin (under maas's instructions, I presume) is trying to fetch root-tgz instead of root-dd (which I confirmed exists in /var/lib/maas/boot-resources/current/custom/amd64/generic/my-image-dd/uploaded).