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.
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.