1. Light a grill. In a medium bowl, toss the rhubarb and shallots with the butter to coat. Stir in the sugar and minced thyme and season with salt and pepper.
2. Tear off four 16-inch-long sheets of extra-heavy-duty foil. Mound half of the rhubarb mixture in the center of each of 2 foil sheets. Gently pound the thicker pieces of pork about 1 inch thick. Season with salt and pepper and press a thyme sprig onto each piece. Arrange the pieces of pork in a layer on the rhubarb. Cover with the 2 remaining sheets of foil; fold up the edges all around to seal.
3. Grill the hobo packs over a very hot fire for about 12 minutes, or until sizzling and puffed. Using oven mitts, transfer the packs to a rimmed platter. Open the packs carefully, transfer the pork and rhubarb to plates and serve.
Notes: One serving: 345 calories, 10.6 gm total fat, 4.7 gm saturated fat, 23 gm carb.