I have a tandem converted to 650B and considered trying front drive as you describe but have not done so. My main concern is the long chain flopping around. From John Allen's article:
If you ride over big bumps, the long drive chain can flop around. Front drive doesn't make much sense for off-road riding. Even on a road tandem, it's a good idea to put a chain idler just ahead of the rear bottom bracket to control the long, slack lower run of the drive chain. But the need for an idler can become an advantage: a sprung, two-pulley idler made from a rear derailleur chain cage takes up chain, increasing derailleur capacity, and improves shifting.
John Heine has mentioned that the old French tandems used a special stronger rear derailleur spring to help keep the long chain from flopping around. This would not really be an option with a modern RD so an idler pulley as mentioned by John Allen might be a needed added complexity.
In the end I decided not to do it because of the long chain flop, added complexity and added weight. The problem of how to add a two pulley idler to a tandem not designed for one is also a problem to be solved.
I expect that the normal rear drive chain probably flops around a lot more than we know but since it is behind the stoker the movement is not noticed. A long chain's flopping would be annoyingly noticeable to both riders.
I still think about the idea because I believe the shifting would be ideal. Please post if you decide to give it a try. I would love to benefit from your experience.