Preheat an indoor grill or barbecue to medium-high. Brush the plums with the 2 tsp. vegetable oil. Lightly oil the grill. Grill plums for 60 to 90 seconds on each side, until lightly charred but still firm. Remove from the heat and cool to temperature. Cut the plums into small cubes, set in a bowl with the other salsa ingredients, and toss to combine. Cover and refrigerate salsa until needed.
For the pork, combine the 2 Tbsp. vegetable oil, chili powder and cumin in a small bowl. Brush mixture on the pork; season with salt and pepper. Grill the pork over medium-high heat 12 to 15 minutes, turning occasionally, or until just cooked through. Rest the pork 5 minutes, and then slice and serve with the salsa.
Note: The plums used in this recipe were about 2" in diameter.