As it has been discussed in a couple other forums here on SLS, I would tend to agree that any such search filter would have more of a negative connotation than positive. I for one, totally get what you are asking to achieve.
I mean, let's say that my wife was interested in finding a black playmate that is extremely well endowed. Put that in the search field, and wallah, a list of potential candidates that she can peruse through and select a name or two to reach out too. Downside is that someone could use that same information for less than honest (or honorable) intentions. As everyone that uses platforms such as this are not exactly as described, never really know what some folks are up to.
In my opinion, continuing with the main 3 choices (men, women and couples) provides an acceptable means and methods for searches. Sure, takes a bit longer to sort through single guys (if that is what you seek) and use a combination of a photo or two and a good description (which unfortunately not all guys subscribe too), in order to determine ethnicity.
As I am neither the operator, moderator or webmaster for this site, I can only offer what I see from the consumer side. And offer my personal comment and opinion.
Cheers