I was thinking about a similar scenario the other day. In which the US, Germany and Holland came out very strongly on the side of the Boers. Now, for a start, the Boers would have to stay within their own borders, and not raid across the line into Bechuanaland and other places. The Germans really couldn't give a damn about the Boers [ from Breaker Morant] but want to give the British a hard time. The Dutch find their errant brethren something of an embarrassment, but in the end, family ties win out over common sense. For the US to weigh in will take some major butterflies, maybe by Teddy getting elected to the presidency in '96, somehow, and by him being more anti-British than he was in OTL. [or have I read too much Turtledove?]
So, In such a scenario, can world opinion force some sort of negotiated setttlement, where the Transvaal and OFS can exist with at least some form of home rule within the empire. I don't know how the average American felt 110 years ago, but in the present era, the Brits would be seen as big time bullies, especially when news about the concentration camps got out.