yes you can do that. we give you full control of your template control and which content it should render based on your data, in your case you just need to read the data from your mongoose collections and send them to jsreport, if your app is using node.js check the available options to do it, if you are not using node.js you need to pass the data via http, for that you can check the docs here.
about layout placeholders check this post, it probably gives you an idea about the options you have.