Now if you really want captain control of the drum using a friction shifter (which has historically been a popular option), the most common drop bar mounting points I recall are either a friction Bar-Con in the handlebar end or a Command Shifter in friction mode inboard of the brake lever, but not interfering with any brifter throw.
Obtaining a Bar-Con or Command Shifter, if one does not have such in the parts bins already, typically involves bike swaps or eBay. And when pondering the asking prices, remember that it's still a small percentage of the price of a new tandem.