in a function panel_name->flags &=~SHOW; // off
panel_name->flags |= SHOW // on

I might not understand what you are asking...

Do you mean it has to render first before you hide the panel? If so set the panel layer lower then the 3d panel and put them in the same place, then the 3d panel covers the 2d panel till it is hidden.

Could you put the 2d panel on layer =1 and a full screen black panel on layer =2 and the 3d panel on layer=3 ?