Hello Andrew,
Ben and a few others here will hopefully nail down the origin of this Mandau. A piece from southwestern Borneo could show up in either British or Dutch collections.
By that time Borneo was firmly under British and Dutch rule. There are early presentation pieces for the Dutch and British crown. Due to the strict colonial administration I don't think it's likely that some local sultan (much less a member of the D(a)yak gentry) travelled to Transvaal or sent emissaries. If something that exotic ever happened, you should be able to find a notice in SA newspapers (I assume the libraries in London will have complete sets).
Apparently, presentation pieces were also given to some local colonial officers. Also early souvenir pieces of lesser quality which were never meant to be used "for real" are known (some of these are now veritable antiques, too). This is the real thing though and my best guess would be that a British officer picked this up while stationed in Borneo (Sarawak), moved on to SA, and possibly presented it as a gift to Paul Kruger - maybe he was more interested in Zulu pieces...
Regards,
Kai