More Info…
Hi Matt.
Here we go (feel free to point any logic/design errors or improvements)… 
That’s my goal:
But, of course I would like to make this on UMG to get all hittests, input events and other features, how you can see it’s an UI Element that will be composed by many UWidgets stacked, and on an inner layer I would need an element that is not part “yet” from UMG…
Some desambiguation:
By UMG Built Geometries I mean the kind of proccess described on ElementBatcher.cpp, lines 150 - 164.
Since none of the FDrawElements::Make<whatever>() predicts such element, I thought about use the MakeCustom, but the only code sample I could get regarding ISlateCustomElement was the Engine FPreviewElement and FDebugCanvasDrawer, both doesn’t draws the polygon I need neither use UMG Built Geometries technique.
ElementBatcher as it is just predicts the EElementTypes drawing rules, personally I think it’s a waste of a so much elegant designed way to draw UI Elements as radar charts, area charts, and so on…
So my suggestion was about “also” get some easy way to just send some direct instructions to fill the BatchVertices and BatchIndices array, and send a FSlateBrush with a texture instead of need to deal with FCanvas, renderthread limitations… I confess that until watching all Epic did, I’m struggling to put MakeCustom() to work. lol
Regards!