No need for another brake in my opinion (which ain't worth much! :) )...
One thing my father drilled into my widdle brain when I first started riding back when I was 12 yrs old: "Keep your feet ON the pegs, toes up!!!". I rarely and do mean rarely put a foot down for a corner, etc... Did this make me a better rider? Tough call... it did keep my feet out of harms way for the most part over the 23 years I've been riding off-road. If you ride standing/crouched and corner with your feet up most of the time, you will not need another brake because your right foot will be there to use the pedal. You just need to compensate with body movement/balance to take your right handers. I actually can't believe how much some of my friend put their feet down around corners... I'm surprised more toes are not missing from them!
Sounds like your riding some really neat stuff! I love when the trail is slippery, nasty, hilly, etc... the worse the better. Also, your front brake can be your best friend in those conditions too, but you've gotta be real 'gentle' with that on slippery/downhill stuff. Try slowing it down a little, keep the feet up and use that rear brake instead of putting the leg out. Have fun and keep it on two wheels!
Matt