|MadSci Network: Biophysics|
You are mixing many physical metaphors in your proposition/question.
A Faraday cage shields against free standing electrostatic fields. It does not shield against electron flow through it. (Also a Faraday cage's effectiveness is not dependent so much on the mechanical attributes of the material it is made out of, just as long as it is conductive, so surgical steel is not necessary to produce a working Faraday cage).
Since an external electrical action upon a person's heart comes about from conduction of electrons through the tissues, a Faraday cage effect would not occur to impede it. The electrons would flow through the mesh much more efficiently than the tissues anyway. Perhaps a metal mesh might distribute the impinging current around the heart instead of any particular spot, but I don't think that's what you had in mind. Finally a steel mesh around the heart, I believe would irritate its action and cause pathology far more likely than any potential positive effect (of which I don't think there is anyway).
Try the links in the MadSci Library for more information on Biophysics.