This is a TODO for LoopBack to support configuration of datasources/models from environment variables and other sources. One idea is to use a template engine to load datasources.json so that it can have variables to be resolved at runtime.
Related to your question, LoopBack allows you to configure the datasource using a 'url' property. For example:
{
"connector": "loopback-connector-mongodb",
"url": "mongodb://localhost:27017/mydb"
}
As a workaround, you can write a post-deployment script for Heroku to replace the url value with process.env.MONGOLAB_URI or process.env.MONGOHQ_URL.
sed -i.bak s/MONGODB_URL/$MONGOHQ_URL/g datasources.json
Meanwhile, please open an issue at https://github.com/strongloop/loopback/issues.