Install a distro from an online repository core make file
Unfortunately the Drush Make infra does not support this use-case well.

The quickest wordaround is to clone the project Git repository and provide a local path to the makefile in the Aegir UI.

For elmsmedia I guess the best makefile would be http://cgit.drupalcode.org/elmsmedia/tree/local.make.example

OpanAtrium has http://cgit.drupalcode.org/openatrium/tree/build-openatrium.make for this same purpose.