Soak 12 long skewers in water for 20 minutes to prevent burning. Thread 4 scallops on each of 4 skewers and 4 shrimp of each of another 4 skewers. Thread 6 mushrooms on each of 2 more skewers. Cut prosciutto into 2x1-inch strips. Wrap 2 asparagus spears together with 2 strips of prosciutto; secure with toothpick. Repeat with remaining asparagus. Wrap each artichoke heart in 1 strip of prosciutto; thread on 2 remaining skewers.
Place skewers, asparagus, zucchini and bell peppers on baking sheet. Reserve 1/4 cup Lemon Baste. Brush remaining Lemon Baste liberally over ingredients on baking sheet.
Grill skewers, asparagus bundles, zucchini and red peppers, on uncovered grill, 7 to 10 minutes until vegetables are tender, seafood firms up and turns opaque and prosciutto wrapped around vegetables is crisp, turning once or twice.
Remove each item from grill to large serving platter as it is done. Pour remaining baste over all. Serve hot or at room temperature.