If you don't mind torque it to the exact values that BMW specifies, you can juse use the flex head box wrench and some regular tools to finish the job. Just tighten the bolt until the mark on the bolt matches it's original position. (you should see a green or yellow line mark on the bolt) Remember to replace the washer.

If you're anal about the torque spec, it would be a lot easier if you have proper swivel adaptor with a super shallow socket like weasel56 said. I haven't found a proper shallow socket though. The shallow sockets on the market I found aren't that shallow enough for this. If one is available, you can access the bolt on the gear from top. Open the hood, you can actually see the bolt.

I found an alternative way to do this. If you take your bumper cover off, and pull out the air duct on the driver side a bit. You can access the bolts on pump and gear very easy with an extension and swivel joint adaptor. It can be done in just about an hour or so. I think that's probably how it's done when car is assembled. Those plastic and metal parts that get in the way were installed after the power steering system was installed during assembly.