A ladies fixie with a tricky brief

The front brake is easy: just drill out the back of the fork with an 8mm bit.

The rear is not so straightforward:

• buy another front brake: the longer bolt will allow you to mount it with a conventional nut, rather than the recessed type;
  
• drill out the back of the tube (as per the front brake) and mount the brake on the front of the tube. Not necessarily a good idea though, as if it's just unreinforced tube (likely on a budget frame), drilling it out will weaken it and it's likely it will distort when the brake is applied;
  
• fit a single pivot, conventional nut calliper: this would be my choice. You don't need the mechanical advantage of a dual pivot on the rear and it would allow you to reverse the cable mounting hardware, so that the cable entered from below as was intended.
  
  

Let me have a rummage, I might have one that you can have for postage.

If you do use a dual pivot, you can use a V-brake noodle to angle the cable: check the CP thread, this came up in the last couple of pages and Ed posted a photo.