If I am not mistaken, you can't put an absorber directly on microstrip. The fringing fields actually extend not only to the sides, but above as well. You will get high loss and your impedance will change. And there are not much thin absorbers. Most of -6dB ferrite based sheet absorbers are narrow-band and rely on normal reflection from ground plane on the backside. In your case it is equivalent to simple screen box. And again, your ground plane actually is not ground... so I think you should avoid using it in the circuitry. I think, you should focus on the circuitry itself, how to isolate and screen it. There are specific on-chip absorbers on the market designed for cpu working conditions.