In his novel "La San Felice", Alexandre Dumas states that Lord Hamilton, having no time to refer to Palermo, wrote a note to the rebels, that could be read as safe-conduct, to induce them to go to the ships. Is there any document or testimony supporting this ?

Of course this is a strange source, but Alexandre Dumas made thorough research to write this novel, out of personal interest. First, his father General Dumas died partly due to harsh treatment in siciilian jail, coming back from Egypt. Secund, the novelist was involved in Garibaldian overthrow of Two-Sicilies kingdom, fitting out a ship in support to deliver weapons, and becoming for two years Minister of Art and Museum in Naples.