To make Filling: In a small bowl combine gelatin and water, let stand 5 minutes. In medium saucepan combine cream, eggnog, sugar & salt. Set over medium heat and stir occasionally until hot. Stir in gelatin mixture. Cool slightly, about 10 minutes, stirring occasionally to prevent gelatin from sinking to the bottom. Remove to an airtight container & chill in fridge until set, or up to 3 days.
To make French Toast: In large shallow bowl or dish wisk together eggs, milk, vanilla, salt, cinnamon and allspice. Cut bread into 3 inch wide slices. With a paring knife, cut halfway into each slice to create pocket for filling. Fill each pocket with eggnog filling. Soak stuffed bread slice in egg mixture.
While bread is soaking, heat a large skillet or griddle on medium heat. Melt butter in pan and brown bread slices, working in batches if necessary. Brown 3-4 minutes per side, or til golden.