Check if mouse is over GUI element using Rendertexture?

My current setup:

I have a game scene, rendered by a default 3d camera. All GUI elements are 3d planes with textures, rendered to the main window, with a secondary orthografic camera.

What I'd like to do:

I want to check if the mouse position is over any GUI element. Not all of them have colliders, so normal raycasting would fail.

My Idea:

Is it possible to use the fact, that the secondary orthografic camera only renders GUI elements and clears with depth? Can I use Rendertextures and check for alpha values at a given xy position, or would that be too expensive? If yes, is it possible to render a camera to the main window AND a render texture?

Thanks for your help!