Set the oven and prepare the pan. Preheat the oven to 350° F, and line a 9 x 5-inch loaf pan with parchment paper.
Prepare the guavas. Cut the guavas in half, and use a small spoon to scoop out the flesh -- you need about ¾ cup of it.Add it to a blender with the entire contents of the can of crushed pineapple, and blend until you have a smooth purée, about 10 seconds on high. You should have 1½ cups of the puréed fruit.Remove ¼ cup of this mixture, strain it, and add it to small saucepan. Set aside.
Make the batter. Pour the remaining juice into a large mixing bowl and add the eggs, brown sugar and vanilla. Stir until you have as smooth a mixture as possible. In a second mixing bowl, evenly combine the flour, baking powder, salt and allspice.Fold the wet ingredients into the dry, fold in the the melted butter, and pour the batter in the prepared pan.
Bake. Place the pan in the preheated 350° F oven and bake until it’s completely solid and doesn't move when jiggled, about 50 minutes. It should be golden brown and cracking on top. Let it cool for at least 30 minutes.
Make the glaze. While the bread is baking, gradually sift the powdered sugar into the saucepan with the guava, whisking as you go. Place the pan over low heat and continue to whisk until there are no lumps, just a minute or so.
Drizzle glaze, slice and serve. Once the bread has cooled to room temperature, drizzle or spread it with the glaze. (You will likely have more than enough.) Slice and serve.