I've since learned how to create the games on demand containers using the official sdk.
QUOTE
Converts a source game disc image file (.GDF or .XSF) to an Xbox 360 secure content file. The output can be manually deployed to a devkit to verify game performance in conditions that would occur when digitally distributed or as a result of game disc installation.
gdf2content -in:SourceImage -out:OutputDirectory [-titleid:TitleId] [-titlename:TitleName] [-thumbnail:ThumbnailFileName] [-q] [-?|-h]
So you would create a game disc layout with the editor, then use that tool above to convert it to a games on demand container which once copied to your 360 HDD will show up under full Xbox 360 games.
You can do it manually if you want, the files you need to insert can be found in quickboot\xbox\ (You will need to change the config.ini with a text editor).
There's currently no
Legal or automatic way of doing this.