Measurement of s11 with reduced power on NanoVNA
Theoretically this would work, but there are some real life limitations.  For instance, with a 20 dB pad, is the expected reflected power still within range of the analyzer?  Also, the return loss of the pad would have to be pretty good (~better than 30 dB) to get accurate results.  

If you have known test units that you could try with and without the pad to see if the results correlate, you could gain more confidence.  A test device with 0 dB would be easy to measure.  A test device with 20 dB would be much more difficult.