How to Make Particles Billboard Non-Square or using custom Mesh?

Hello, we're doing our best to reduce the Overdraw of the Effects Rendering in our game. We noticed that Particles of the particles systems (both the classic one and the new one) can only be rendered as Square Planes when standard billboard effects are required. So it seems to impossible for us to save the almost transparent pixels in particles specially the texture in round or cross shapes. (as alpha test and clip() is not suitable for mobile platforms). Any advice for us?