You’re going to need to do that inside the component visualizer. The selected spline points aren’t “real” objects, they’re hit proxies which will be intercepting user input in the editor and interpreting that data to manipulate the spline’s array of spline points directly.
Give this a read for how they work:
I assume your plugin already has an editor module. What you’re going to need to do is create a new FSplineComponentVisualizer, unbind the default visualizer for the spline via GUnrealEd->UnregisterComponentVisualizer in your module’s startup code and then bind your new version with the additional functionality. You might also need to tweak your plugin load order to ensure that this happens after the normal binding, but I’d expect that to probably be okay by default.