Hi,
I love Greensock, but the reason why DOTween's code is not lose scripts is tied to the fact that namespaces and the internal keyword work very differently in ActionScript and C#. I initially wanted to make DOTween as lose scripts, but a decent API would've been truly impossible (not to mention the speed of compilation and other features that a pre-compiled DLL allows).
If you're curious about DOTween's code, its open source is
here. Though indeed it's missing the Pro part, which I wouldn't know how to share right now.
That said, if you want to do the stuff you wrote, you'll have to get the DOTweenPath's tween (myDOTweenPath.GetTween()) and use the open source libraries with it:
// Get the tween
tween = this.GetComponent<DOTweenPath>().GetTween();
// Add a callback to know when a waypoint is changed
tween.OnWaypointChange(WaypointChangeCallback);
// Set the playhead to a specific waypoint
tween.GotoWaypoint(2);
// Set the playhead to a specific time
tween.Goto(2);
You can find more info about those methods
here.