You can still Bo-Duke-en while super sprinting if you press the carjack button much sooner than you would have in SR3, I'll say about a car and a half width away from the target. You can hammer on the button as you are running towards the vehicle and it should still go through when you get in the correct range.
Same way you perform Super Beatdowns - run directly at it.
The super "force" of your Super Sprint only affects your side and behind you. Sprint directly at your target and spam the button, you should find it works.
I'll agree it's annoying you don't have a button to toggle Super Sprint, sometimes I want to use the normal sprint because it's just the right speed for quickly moving, but nope can't even use it when you want to lol.