Preheat oven to 325. Place roast on rack in a 9x13 baking pan. Roast, uncovered, for 1 1/4 hours or until temperature registers 150F.
Meanwhile, in a medium saucepan combine rhubarb and juice concentrate. Bring to boiling; reduce heat and simmer uncovered for 10-15 minutes or until rhubarb is very tender. Strain mixture into a 2-cup measure, pressing out liquid and discarding pulp. If necessary add water to equal 1 1/3 cups.
In the saucepan, stir together the cornstarch and cold water. Stir in the rhubarb liquid. Cook and stir on medium heat until thickened and bubbly. Reduce heat and cook, stirring for 2 minutes. Add honey, mustard, and vinegar. Heat through.
Brush some of the glaze on the meat during the last 30 minutes of roasting. Cover meat with foil. Let stand for 15 minutes. Temperature after standing should be 160F.
In a small saucepan, bring the remaining glaze to boiling; boil for 1 minute and serve with meat.