tl;dr: If you want to attract diversity to your company, and candidates get interviewed by N people, then I think you need your employee population in any given job title to be 1/N diversity.

I've been musing on creating a workplace that underrepresented populations want to be. I'll use women as the example.

Say in your industry, there are a lot fewer women than men. Say its due to known sexism an mistreatment issues that have been well documented(1). Lets postulate that not ALL people behave this way, but some percentage of people do, and the general industry has enough people who do behave this way that it has obviously driven out a lot of women.

If your company has a goal(2) to create a non-hostile work environment for women, then presumably you don't want to hire those people who drive women out! 

However, "treats women poorly" is something you only have a chance of catching at interview time if you have at least 1 interviewer who is a woman, so that you can observe how the candidate treats her. This isn't a guarantee; she might not notice bad behaviors that do happen, the candidate might not exhibit bad behaviors in the interview. But, we're going for "lower bound" math. Obviously, redundancy is better.

If you dip below 1/N women in your company, then you've got an expected value of less than 1 woman on any given interview slate. As a result, you'll start to bring in people with the traits that drive women out until you reach the jerk-rate of the general population (which we've defined as "too much" above). Over time you'll have major attrition problems unless you take drastic action to counteract the situation.

Most drastic measures I've heard aren't reasonable to run in the long term, and are maybe even unreasonable in the short term:

-- If you want to treat your employees fairly, they should be on interviewing duty equally, since interviewing normally doesn't count towards promotion and its a lot of work. So "overload the women with interviews" is a non-option if you want them to get promoted at the same rate as men. And you do want this if you want an environment where women and men have the same chance to succeed (i.e. a non-hostile work environment).
-- Likewise, you can't ask men to change their gender identity for 45 minutes. Gender identity doesn't work that way, and this would be harassment of the men, which is not remotely ok. 
-- If you lower the hiring bar for women (or raise it for men), then you will end up in a situation where people's pattern matching neurons start to associate women with being less capable at the job. Which will lead to the behaviors and unconscious bias that we're trying to avoid.
-- Find some kind of non-promo relevant busy work to give to guys so that women and guys are equally taxed with "community service" tasks, even if the average woman does more interviews than the average man. The problem is that if this is remotely interesting, desirable work, then its unfair that the women don't get a choice. And, if its completely mundane and dull, then its gonna piss off the men AND waste company time.
-- Fire people until you get to 1/N women. This is bad, and I don't think I need to explain why.

The best drastic measures I can think of are:
-- heavily market job opportunities towards women. Write your job reqs in a way that attracts women.
-- hire interviewers who do nothing but interview, and have 1 of these on a slate of N. These people could have a lower technical bar, because they only need to learn a few interview questions really really well. They still need to be technically competent. If you do this, you could have employees who do nothing but interview and write feedback all day, so you wouldn't be overtaxing any of the normal engineering employees. But this might be illegal for 14 different reasons, I'm not a lawyer. Just trying to engineer my way out of this hole.
-- raise the number of interviewers a candidate sees until N is big enough that 1/N meets what you've got. This is a waste of engineering resources, and might exhaust candidates badly, but at least it isn't immoral.

So. All the drastic measures are awful and not sustainable. I think the take away then is don't fall under 1/N women in your company in any particular job type. If you can get to 1/N, then it might be possible to bootstrap to an actually good ratio (i.e. near 50% in the case of women). But below 1/N, it sounds like at best your company will lose women until the remaining amount are those who aren't bothered by the bad behaviors.

(The math works "in reverse" too. Don't fall under 1/N men.)

(1) http://www.uchastings.edu/news/articles/2015/01/double-jeopardy-report.pdf

(2) There are lots of reasons to have this goal. From what I understand, it is a legally required that you try, in the US. Diversity has well documented benefits: http://www.theatlantic.com/business/archive/2015/01/the-secret-to-smart-groups-isnt-smart-people/384625/
Also, its hard to hire enough good people right now, so if there is some untapped market of smart employees, presumably you want to capitalize on that.