several options:
They make these bar end brake levers just for that purpose. Some are meant to run the cable inside the bar and some outside on the underside of the bar

Or, it you need to run barcon shifters at the end, you can use interrupter levers:

Or, as you said earlier- just turn the lever around. This works better on bull bars that have a flat top and a distinct drop section (instead of sloped shoulders):

^ In this configuration the brakes worked extremely well. The aero brake cable routing goes really smoothly along the bends. Whereas if you turned the levers around and force the cables to double-back on itself, I am sure the braking would be way worse.
Yet another option , is to run non-aero brake levers. Shimano did make non-aero levers, which were compatible with SLR brakes:

Let's see the rest of that bike!!!!