Not that implausible.
The biggest mistake of the Boer Republics was striking first. While raids with Commandoes work very well when your opponents are Bantu tribes, it doesn’t work so well when your opponent is the British Empire. Had the republics taken an entirely defensive footing and appealed at the same time to the European public their prospects would have been better. There was a lot of sympathy for the Boers in Europe during the war, not just in Germany. Had independent journalists been allowed to observe the war the treatment of the Boer civilians would have outraged Europe.
Direct intervention is difficult because the Boers have no direct access to the coast. The only way to the ocean that is not controlled by the British is via the Portuguese colony of Mozambique and while the Portuguese allowed some arms imports via their territory prior to the war starting, they were an old British ally with neither the strength nor will to intervene.
I doubt any of the other European powers would have directly intervened but they could have done so indirectly or, more importantly, they could have forced Britain to accept a cease-fire and arbitration with the Boer Republics with would have resulted in them keeping their independence.