I have been able to install repositories that do not have a composer.json file like this:
{
\"type\": \"package\",
\"package\": {
This is simply a missing feature. There should probably be a new type of dist which is just a single plaintext file to be downloaded and left as-is. Please file a feature request on the github issue tracker: https://github.com/composer/composer/issues/
EDIT :
The feature actually exists but wasn't documented.
"type": "file"