Dear Gwyn Norman Crump,
There was Johann kaspar Velthusesn, who served as minsiter at the Royal court in London, but only 1770-1773, see here:
Johann Kaspar Velthusen – Wikipedia. On hom, you can find several documents in the state archives of Lower Saxony (Staatsarchiv) in Wolfenbüttel and Hannover, as well as in Stade ; you can search these in AIDA online (http://aidaonline.niedersachsen.de/, just search for "Velthusen") and then contact the archives for copies. None of the hits has an obvious relation to the US, but you never know; there are some letters and two versions of his last will among the records and who knows what he wrote there.
However, here is a reference (Die Auswanderung Aus Dem Herzogtum Braunschweig Im Kräftespiel Staatliche ... - Cornelia Pohlmann - Google Books)
that points out that he published his letters from and to the reverends Roschend and Storch in North Caroline in print in several volumes ("Nordcarolinische Kirchennachrichten") and you can easily find references to these (and even digital versions of some) when searching in Google, so I assume you know these? Here is NC reference to him: http://www.ncpublications.com/colonial/Bookshelf/lutheran/intro.htm
regards,
Nils (Lenke)