While the South African rugby team has a far fairer ratio of black and white players compared to the NFL, over the past two decades it has shifted significantly from an essentially all-white team when they returned to international competition in the mid-90s (post-Apartheid).
At the 1995 Rugby World Cup there was only one black player in the squad, Chester Williams (whose position was wing, the equivalent of a WR/CB in football). This was probably an artificially low number of non-white players, as ethnic groups such as the Cape Coloureds always had strong rugby participation.
Basically ever since 1995 though, the Springbok team and its racial make-up has become extremely political. Rugby is huge part of white Afrikaner culture in South Africa, and particularly in the Apartheid days the Springboks were seen by opponents as being a symbol of the regime.
While there were a couple of failed attempts from politicians in South Africa to remove the Springbok emblem and replace it with something else, for the past 20 years there has been a fairly consistent push towards "transformation" in the national team - with the goal of bringing representation in the squad in line with South Africa's demographics.
This has resulted in a quota system which specifies there must be a baseline percentage of black/non-white players in both the starting line-up and extended squad. Although some of these players are certainly deserving, there have been and continue to be instances where white players are forced out of the starting team or the squad to make room for a non-white player who is simply not as good.
Despite this meddling, the Springboks have performed remarkably well, and even won the most recent Rugby World Cup in Japan. However they can be a wildly inconsistent team, as illustrated by their record against their biggest rival - New Zealand. While before the 1990s South Africa had a slight edge in head to head victories (dating back to the 1920s), over the past three decades New Zealand have won about 70 per cent of their fixtures against South Africa.
I don't know of any South African rugby players who have had a go at playing American football, but there are certainly a few examples of the children of immigrants to the USA having success. Former Steelers guard David DeCastro is of South African descent - his father played rugby at the University of Cape Town. Linebacker Nate Landman, who went undrafted in this year's draft, is the son of Shaun Landman, who played rugby for Zimbabwe in the 1990s.