Another possibility would be to use the Effects>AO shader in your floor material’s diffuse channel instead of using the render effect. Or … you could use the post effect and enable the “evaluate transparency” option. But that may result in too little effect. Or … you could use the post effect but cheat a bit by adding a white knot at the far left of your AO gradient and moving the black knot to the right, probably somewhere between 5-10%.