It seems fairly certain that the following are US citizens (in addition to any mentioned in previous posts):-
Sir Douglas Alexander (1921) 3rd Bt b 1936 (Florida)
Sir Douglas Booth (1916) 3rd Bt b 1949 (California)
Sir Michael Brownrigg (1816) 6th Bt b 1961 (California)
Sir Francis Bruce of Stenhouse (1628) 12th Bt b 1926 (California)
Sir James Dunbar (1694) 14th Bt b 1950 (South Carolina)
Sir Daniel Duntze (1774) 9th Bt b 1960 (Missouri)
Sir Ralph Graham (1629) 14th Bt b 1950 (Tennessee)
Sir Kenelm Guinness (1867) 5th Bt b 1962 (Maine)
Sir Thomas Johnston (1626) 14th Bt b 1956 (Alabama)
Sir Keith Munro of Lindertis (1825) 7th Bt b 1959 (Vermont)
Sir Rowland Murray (1630) 15th Bt b 1947 (Georgia)
Sir Derek Ryan (1919) 4th Bt b 1954 (Idaho)
Sir Charles Seton (1683) 13th Bt b 1948 (Florida)
Sir William Style (1627) 13th Bt b 1945 (Wisconsin)
plus the following less certain but possible:-
Sir Anthony Buzzard (1929) 3rd Bt b 1935 (Georgia)
Sir James Forbes (1823) 8th Bt b 1957 (California)
Sir Charles Grove (1874) 5th Bt b 1929 (?)
Sir Richard Sullivan (1804) 9th Bt b 1931 (Oregon)
Sir Jason Whitmore (1954) 3rd Bt b 1983 (California?)
Sir John Wiseman (1628) 11th Bt b 1957 (Massachusetts