Here's a different perspective: it's easier to do this with bar-end shifters than integrated shifters. With integrated shifters, the brake pull and the left shifter pull are too short. You can get drop-bar brake levers with V-brake pull (Tektro RL-520), and the left shifter will be friction instead of indexed, so it can run any derailleur. Burley, for instance, (before their reorganization) used to sell flat/drop conversions for their tandems that consisted of these parts.