Extending dog leads

AFAIK, no.

If one were to be made, I have very slim hopes of it getting any real consideration.
In the view of the general public, the normal, expected and approved kind of cycling is casual cycling.
You know, riding at a pace that would allow you to enjoy an ice cream cone held in one hand while steering with another.
At the speed where this is possible, errant dogs, Latte moms with prams two or three abreast, iPhone/iPod zombies arcing across your path in a Brownian motion all become easily negotiable obstacles not even worth to mention and even less worthy of corrective action from society.

If you insist on travelling with some kind of efficiency, or at an elevated heart rate, then you are the one displaying deviant behaviour and should in no way be encouraged in your delusions.

Be grateful for sites like these where you can meet kindred spirits, also dreaming about being able to ride at pace, in traffic, as a part of everyday life.