If you had a known problem mode of narrow bandwidth, you could attempt to turn it into a pressure/resonant/membrane absorber. Perhaps using pegboard covered in fabric. Would have to identify a potential frequency and go through the design calcs. Not a ton of area so would have only a mild effect, but concentrated in a narrow bandwidth vs spread out as a broadband velocity absorber, perhaps that might have a more audible final result?
Not sure. Worth a thought.