1. Prepare rhubarb: Bring sugar and 1 qt. water to a boil in a large saucepan. Reduce heat and simmer 3 to 5 minutes. Remove from heat; add hibiscus, zest, and grenadine; and let stand, covered, 10 minutes.
2. Meanwhile, split rhubarb lengthwise if needed so stalks are 1/2 to 3/4 in. wide. Cut crosswise into 1/4-in. slices.
3. Strain hot syrup into a bowl, then return liquid and zest to pan (discard hibiscus). Reheat to simmering. Remove from heat and stir in rhubarb. Cover and let stand until rhubarb is just tender, 1 hour. Uncover and let cool. Chill, covered, until cold, at least 2 hours. Strain just before using, saving liquid for lemonade or cocktails.
4. Make crumble: Preheat oven to 325°. Beat butter, salt, and brown sugar in a bowl with a mixer until evenly blended. Add flour and almond meal and beat until mixture comes together. Beat in nuts. With your fingers, break up mixture into 1/2-in. clumps onto a rimmed baking sheet. Bake until light golden, about 20 minutes. Let cool.
5. Make Chantilly: Put cream, mascarpone, sugar, and zest in a mixing bowl. Scrape vanilla seeds into bowl and discard pod. Beat until as thick as whipped cream.
6. Serving: Spoon 1/2 cup Chantilly into each of 8 widemouthed glasses. Spoon 2 to 3 tbsp. crumble, then 2 to 3 tbsp. drained rhubarb onto each (you may have leftovers). Garnish with lemon, if you like.
*Find dried hibiscus (also called Jamaica) at Mexican markets or at worldspice.com (it's listed with leaf teas). Find grenadine syrup with cocktail supplies. Find almond meal, such as Bob's Red Mill, in the baking aisle at well-stocked supermarkets.
Make ahead: Rhubarb up to 3 days, chilled; crumble up to 3 days, airtight at room temperature.