Comment 1 for bug 1568918

The linked branch lp:~smoser/maas-images/trunk.hwe-x does the needed config changes in maas-images.
You can build your own maas-images and point your maas at fairly easily.
To build:
  # prep system for building
  bzr branch lp:~smoser/maas-images/trunk.hwe-x
  cd trunk.hwe-x
  ./system-setup

  # build for specific arch / release
  burl=http://cloud-images.ubuntu.com/daily/server/
  arch="amd64"
  rel=trusty
  serial=$(date --utc "+%Y%m%d")
  meph2-build amd64 trusty $serial $burl/$rel/current/$rel-server-cloudimg-$arch-root.tar.xz $rel-$arch-output.d

given the above, you'll then have a directory named
  trusty-amd64-output.d/

And you can point maas at that as ephemeral image source. Note, because it is not signed, you have to provide the full path to the index.json file (<PATH>/streams/v1/index.json). I'm not sure off the top of my head how maas merges multiple streams or what happens with that, but this *will* produce a stream with trusty and hwe-x kernels.