The new Xara Designer Pro X (365 or version 12) now works properly with VPX and MEP for creating overlay animations, finally! It was broken in versions 10 and 11.
In MEP 2016 Plus/Premium, right-click on a video object, select Video effects from the menu, Create overlay graphics/Animations, and the video opens in Xara. Create an object, like a star, give it a name, click on Copy in the Animation box at the right to advance (0.5s by default), move the object, Copy, move, repeat. You can try changing the size of the object, colour, rotation, even the form and everything will tween. You are subject to the Flash animation tools, so feathering and the fancy new brushes cannot be used. File, Exit and return to document, and the animation will be brought into VPX/MEP. Double-clicking on the animation re-opens Xara for editing.
You can create multiple objects and animate them. This is the compositing tool that we needed. If you want moveable, resizing masks, make sure to make them white. You can use transparency, but not feathering (which is an annoying limitation).
Note that the animation created, so far as I can tell, will be the same length as the clip, so trim the clip to just what you want. Note the length of the clip. In the animator, you have to continue it to the end or the animation will start repeating in MEP until the end of the clip.
Warning, I tried using the new brushes and they cause both programs to crash.