In-circuit is hard, but you might take a single part of a DUT and embed it into two "matching" circuits on the ouput and input side. Considering power levels, Use your metrology-grade RF source, let it fire into the input matching circuit, thru your DUT, the output-side circuit and use a really good e.g. rf level meter (e.g. two dummy loads with a temperature sensor bridge, supposing that equal effective voltage gives equal temperature, or look into bolometers, whatever, or a VNA/SNA with suitable specs), then remove the embedded DUT and see what RF behaviour you get. You might be closer to your goal then.
BR
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