Cook rhubarb with honey in a medium saucepan over medium heat 6 to 8 minutes, stirring occasionally, until rhubarb is very soft and just starting to fall apart. Preheat oven to 350°.
Remove pan from heat and add vinegar and peppercorns; let sit at room temperature (so flavors meld) at least 1 hour.
Slice baguette meanwhile on the diagonal into 1/2-in.-thick slices to make Put slices on a baking sheet, brush one side with oil, and bake until golden brown, about 15 minutes. Let cool.
Cut cheese into 26 thick slices, each about as long as a baguette slice.
Spoon a scant 1 tbsp. rhubarb mixture onto each baguette slice. Top each with a slice of cheese (or two smaller pieces) and serve immediately. Or serve toasts, topping, and cheese separately and let guests assemble the crostini.
Make ahead: Toasts, up to 1 day, stored airtight at room temperature; rhubarb topping, up to 1 day, covered and chilled.
Note: Nutritional analysis is per crostino.