Dan is correct but it doesn't answer the question. After much thought, I have finally figured out how to do it, only thanks to Dan's answer.
The problem is that a subsequence is one-dimensional and does not therefore include any of the additional information which is extractable from the three-dimensional structure (when available). Obviously, having the three-dimensional information is much more informative and descriptive for building classifiers, predicting interactions, interaction stability, or other purposes.
Find all intramolecular contacts within a radius of N. for the Amino Acid subsequence of interest (such as an Interface). I set N to, e.g. 4 Angstroms.
Calculate the descriptive statistics based on the set of hydrophobicity values for all intramolecular contact atoms or amino acids (depending on the granularity). e.g. Make an average of all of the hydrophobicity scores from the selected scale. (I use GRAVY, although there are many others).
To improve this further, the distance value of N can be set more intelligently than as a static value. For example, to the same as the cavity radius.