How do you render a non CustomUI object above a CustomUI?

I want to display a particle effect over the CustomUI but I'm not able to because the CustomUI renders above everything even an object that is closer to the camera. The CustomUI is an overlay.

Z-index is available but that only applies to elements within the CustomUI. I want to show an separate object that's NOT a CustomUI.

What I've tried already:

• Attached the particle effect on the head anchor. Result: CustomUI still renders above everything
• Attached both CustomUI and particle effect on the head anchor. Result: CustomUI won't be an overlay anymore

Again, my question is: Is there a way to render a non CustomUI object above a CustomUI (overlay)?