Burried via with custom shape in Altium

Does anyone know how to implement in Altium a buried via (a via in PCB inner layers) with e.g. rounded rectangle shape? E.g. i could define a pad with rounded rectangle shape and connections in inner layers, but the pad hole can go through full stack only.

I need this for a very specific design, and from manufacturer point of view this could be done. But, I would need advice on how to specify it properly in PCB design.

Perhaps there exists any other methods how to achieve this (specify a rounded rectangle shape slot with plated sides in PCB inner/core layer) with other methods?