Is there a way to run a jsx file from within a plugin?

Here is JavaScript for playing JavaScript. You can convert the ActionDescriptor code to C/C++ and run it in Getter or Listener example.

var id4 = stringIDToTypeID( "AdobeScriptAutomation Scripts" );

    var desc2 = new ActionDescriptor();

    var id5 = charIDToTypeID( "jsCt" );

    desc2.putPath( id5, new File( "C:\\Users\\tr\\JavaScripts\\HelloAlert.jsx" ) );

    var id6 = charIDToTypeID( "jsMs" );

    desc2.putString( id6, "hello" );

var a = executeAction( id4, desc2, DialogModes.NO );

There is also the JavaScript suite you can acquire in an automation plugin and execute JavaScripts that way.

Not sure about the return values. You might have to store/retrieve from the Photoshop registry but I think the last thing you have in your script should get returned in the ActionDescriptor from the play command in C/C++.