Hello Valthoron,
After looking over your issue I was able to find a workaround for the gimbal that you would like to make. You were on the correct path. I was able to create room that held the 3D object that I wanted to display and from there I used a Render target 2D in combination with a Scene capture 2D camera for the effect. I have provided the example below. I hope that this helps.
Example.
I created a room that I applied a flat black material to and placed my 3D object inside of it. I then aligned my Scene Capture 2D camera so that I got the angle that I wanted. Please Note: That if you are going to place the fake room below the level as I have done in the example, you will need to move the kill volume down so that it does not destroy your actor.
I then created the materials that I would need. These Include the flat black that was mentioned above, the cube color that is an unlit emissive, and the masked material that uses the render target 2D texture.
This is a picture of the final result. The cube is rotating in place and it has been added to the viewport view an image widget that has been applied to a widget blueprint.
Make it a great day