Not a marinade, but this is a nice sauce for pork. The recipe uses a pork tenderloin, but I'm sure it can be modified for a roast.http://www.pillsbury.com/recipes/ShowRecipe.aspx?rid=14331
Pork Medallions with Dijon-Mushroom Sauce
5 teaspoons butter
1 shallot, finely chopped (2 tablespoons)
1 jar (4.5 oz) Green Giant® sliced mushrooms, drained
1 teaspoon vegetable oil
1 pork tenderloin (1/2 to 3/4 lb), cut into 8 (3/4-inch-thick) slices
1/8 teaspoon salt
1/8 teaspoon pepper
1 tablespoon cognac or apple juice
1 tablespoon Dijon mustard
1/2 cup whipping cream
Fresh thyme sprigs or basil leaves, if desired
1 . In 8-inch skillet, melt 2 teaspoons of the butter over medium heat. Add shallot and mushrooms; cook and stir 1 minute or until tender. Remove from skillet.
2 . To same skillet, add remaining 3 teaspoons butter and the oil. Add pork; cook 8 to 10 minutes, turning slices halfway through cooking, until browned and no longer pink in center. Sprinkle with salt and pepper. Remove from skillet; cover to keep warm.
3 . Gradually add cognac to same skillet; stir in mustard and cream. Heat to boiling; boil 1 to 2 minutes, stirring constantly, until slightly thickened. Add mushroom mixture; cook 1 minute or until thoroughly heated. Serve sauce over pork slices; garnish with thyme.