Q. It occurred to me that even those (Poskim), such as Horav Shlomo Miller, who are meikel (lenient) regarding sherry cask whiskeys, would think it appropriate to be machmir (stringent) in a situation where a non-Jew or an intermarried couple is present?

A. It would not make any difference since as is, the original sherry imbibed and absorbed on the cask’s wood and also the liquor stored there after, were not guarded or sealed. See question 1748 above.
Horav Shlomo Miller Shlit’a mentioned that the prohibition of not drinking at a meal or in a bar with a Gentile, that pertains to all kinds of liquor, may apply here too (See Shulchan Aruch Y.D. 114: 1).

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a