challenging question as always 😉
I have a question about your exact use-case: Is it really just rectangles with borders that you need, or would you also need sprites?
What you are looking for, could probably be done by adding tinting (dynamic change of colors of sprites) and 9-slice sprites aka 9-patch sprites. You could implement the 9-patch technique by yourself by placing images with a Grid element and scaling the inner part appropriately. But for the sprite tinting we would need to add support for it to V-Play.
If you rely on rectangles (which have a worse performance than batched sprites though, because they are not batched and each rectangle requires an own draw call) there is currently no way to get the rounded borders. We are working on an improved clipping item though, which also supports a texture and not only rectangular area. But you would still require a 9-patch texture then so the borders look sharp on each platform.
I would rather go for a 9-patch sprite with tinting as described above (which we both need to add to the engine first), but if you could describe your use-case more another solution might be better!