Mid-bar brakes for only rear wheel?
If you were going to only add one set of cyclo/interrupter brakes to your drops, I'd add them to your front brakes:


Front  brakes are more powerful and provide almost all of the braking force in emergency stops (i.e., you apply so much braking force you are almost in an endover situation). 
Rear brakes skid out more


So assuming your hands are on the tops of the bars when that car pulls out in front of you, you want the set of brakes that actually work to work when you squeeze them.

The cons are that if you're a newbie, the front brake is harder to modulate. The easy solution is to gain more skills in that area.

The other con is that Americans inexplicably have the front brake on their left hand. You can easily (on many bikes) flip the handedness so that your dominant hand (usually right) controls the front brake. I flip all my brakes (right-rear) because I'm used to riding a motorcycle.



p.s. This photo shows an interrupter on the right hand side going to the rear brakes.