Trekking bars are one of those good-as-far-as-the-name-goes products. Simple fix would be Avid BB7 mechanical disc on the front, and run your current lever on the tops of the drop bar to the rear. That way when you are diddling along on the tops you can run off the rear brakes, and when you are maxed out on a hill on the drops you have the front brake. You could add a redundant lever to some cantis on the rear from the other drop position. This gives you for not too different weight what people are acheiving with cross brakes at the top yet you have some real parts and mechanical redundancy.
Or just go all BB7. Where discs are used on touring bikes mechanicals tend to get the nod for easier repair etc...