I don't really have insight on the BOP. It seems to me that a lot of agencies have more non-litigation reasons they need attorneys, and I could imagine a bunch for BOP because there isn't an administrative, article I court for prisoner appeals-I think they all go to article III courts. My best guess is that they need in-house counsel for employment issues and to refer suits against BOP employees to the DOJ. They probably also do troubleshooting and internal legal memorandum. In DC, you'll probably find a higher level of advisors and people who do policy work. Could actually be interesting stuff, if you are into institutional sociology. Again, this is all speculation, but it gives you an idea of what kinds of things you need to figure out.
As far as I know, there is no federal affirmative action. I do think that the government, as any employer would, seeks out diversity because it makes a better workforce. They do recruit at minority job fairs as well. That being said, the minimum standards, as far as I can tell, are not lowered. The government a pretty diverse place attorney-wise, relative to the population of top law students generally (which seemed horrendously un-diverse at my school), and the minority attorneys always have exceptional credentials (like everyone else, really).
There is no need to assume people have to choose government work. It would be the incredibly rare person who could do nothing but work in government. The quality of the private job may be in question, but, at least until recently, you could always pass on the public service and either hold out for something in in private practice or take some other job offer. There would be a question as to whether it would be a good idea to pass it up, though, if you have no current offer. I didn't have any offers at the time that I took the government job, but I had 4 or 5 interviews lined up, which I cancelled once I got the government one. When it came down to it, I was not very happy with my summer in private practice, and I was left with this brief feeling of fear, not because I wouldn't find another job, but because I felt like I didn't even have a good skill set from law school. At some point, I resolved to getting a job that would give me a specialty and a lot of training fast, so that I knew I would never have to worry about not being competent. Experience-wise, there is simply no better way to start as an attorney. I mean, once you stop worrying about the money, government work is a no-brainer.
The other thing was that I felt I could work in private practice anytime, but the government opportunity seemed more difficult to come by. Not that you couldn't find a government job, but later on a lot of things can get in the way, or competition may be stiffer. Ideally, at the time, I would have worked for a couple years in private practice to pay off loans, and then gone to govt (that was the plan). But, when the perfect job comes up, I think you have to jump on it even if the timing isn't perfect. I mean, timing is like 90% of getting a job anyway. They wanted me now, I couldn't guarantee that they would want me later, so I got it while the getting was good.
It makes me look like a genius now. I'm watching friends apply for similar jobs in the last year and get denied, even though their grades were better than mine. Their timing is just really bad. One of them passed on a DOJ job two years ago because it wasn't the exact group within the section he wanted. Now he can't even get an interview (something tells me they may remember when you turned them down, too, though).
If you are unwilling to work long hours and push yourself to the brink, avoid the DOJ or USAO's. These jobs are for overacheivers. Again, you'll find the best hours in the agencies. Also, with that attitude, definitely forget about getting appellate work. It is 100 times more competitive, so you need to either build the resume of an overacheiver, likely including both a DOJ/USAO trial attorney experience and a high-level clerkship, or clerk on the supreme court. I'm sure these jobs can be had through other means, but everyone would love to get all the prestige of doing appellate work only and avoid trial work, so it is incredibly competitive. If you do criminal work you handle your own appeals in the USAOs.