I am grateful to Mark Anderson who found reference to Trollope in the diary entry of Irish poet William Allingham for 26th May 1868. Allingham was visiting his friend Robert Browning who showed him some new works including an idea based on the trial of Count Guido in Italy. Browning said that he had discussed the idea with Trollope with a view to Trollope writing a novel based on the storyline but Trollope had declined. Browning had therefore decided to do something with it himself.
Who knows what Trollope might have made of the story? His only work that is based on historical fact, La Vendee, met with little success so perhaps he was wary of writing about real events. Perhaps by this stage Trollope knew that his strength lay in creating imaginative fiction set in an English milieu?