You could run hydros down a steer no problem, my issue with them was that the stock magura lever just feels like garbage. My trials bike has these Echo levers on it and they feel awesome. I have been thinking about going hydro on the BMX again.
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Although BMX bars might not be wide enough for that lever, on my trials bike there is about 2" of exposed handlebar between the brake clamp and grip.
I don't like running dual cables straight into the brake arms, I don't think the cable angle ends up right and you lose a lot of power. That little adapter is some thing SST made but only ever sold on completes, essentially turning a dual cable set up into a straddle and getting a better pull angle. Its also the same principle as the people that cross there dual cables over one another before going to the brake.