You can render the 2D UI into a RenderTarget::Image texture on your Camera2d, then apply that texture to 3D objects. See the Bevy render_ui_to_texture.rs example.
https://github.com/bevyengine/bevy/blob/main/examples/ui/render_ui_to_texture.rs https://bevy.org/examples/ui-user-interface/render-ui-to-texture/