Blend drained roasted peppers with the 2 cups broth until smooth. Put drumsticks, onion, garlic, oregano, salt and pepper in a 6-quart slow cooker and pour over the pepper mixture.
Cover and cook on low for 6 hours until chicken reaches 165°F and the meat pulls easily from the bones. Lift the drumsticks onto a tray and leave the cooker covered while you remove bones, gristle and tendons.
Strain the cooking liquid into a heatproof measuring jug, pressing the soft vegetables lightly. Inspect the retained vegetables for any stray bone fragments, discard any fragments and measure 3 cups liquid, adding extra broth if needed; return the measured liquid and only the checked soft vegetables to the cooker.
Turn to high, add dry penne and the cup of milk and stir so the pasta sits in the liquid. Cover and cook for 30 to 40 minutes, stirring gently after 15 and 25 minutes, until the penne is tender.
Fold in the boneless chicken pieces, cover and heat for 5 minutes until the meat is 165°F. Switch off and stir in mascarpone until smooth, then fold in parsley.
Divide among six bowls and serve promptly while the sauce is loose enough to coat the penne. The dry pasta cooks entirely in the measured slow-cooker liquid.