Alas, Schiller can't have, because he died five years before they were published. (In 1810 - 52 years after her death -, Schiller died in 1805; Wilhelmine was careful not to have her tell all published within her or her brother's or her husband's lifetimes.) Otoh, I suppose it's just possible that he could have read them in manuscript if he really wanted to, because he did know enough nobles and librarians in the relevant principalities.