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I've seen a few iterations (can't find any right now). They're usually pretty bad. PPL literally clamping a shifter onto the front of a brake lever. The tricky bit is the outer cable holder (whilst machining would be ideal you can be quite precise with a hand file given time) but as lots of MTB or budget road bike shifters have these built in it seems common to use one of these instead and not DT shifters.
Yep I use them on my cx/ beater/ tourer and love them.
Really reliable, comfortable. Thinking of doing exactly as you are and getting a set for my tandem too.