Rory, could this be a misprint of somewhere like Waboom?
On the Forum we recently heard about a burger who fought at Heiwand and Jaline, both of which I think must be mis-prints. Do you have access to a scan of the handwritten original?
The past is not dead. In fact, it's not even past.
According to SAFF, JMR had two casualties at "Wandoom" on 1st April; an "88 Tpr. T Regan" ( Severely wounded) and your man. Very likely a clash of patrols or an ambush of scouts. I note in Stirling that in March and April (1901) JMR had two companies in Colonel Colville's column (on the Standerton line) and did good work at Roberts' Drift on 30th March.. Another part of the Corps was on the Zululand border "and saw much arduous service".
Two areas in which perhaps to look for "Wandoom". Otherwise not much help, I fear.
After a trawl through the newspaper archives I did come up with the attached which, sadly, doesn't put much meat on the bone.
The problem is Waterval has many and varied connotations - the Boer POW camp, Waterval Drift (which seems to be in the OFS), Waterval Boven etc. The closest I came to finding more was the reference to Robert's Drift but this too seems to lack any mention, geographically, in the history books.
The Register shows only two casualties in the rolls at "Wandoom" as Ian notes for the JMR.
I wouldn't assume 'Wandoom' is a gross mis-print for 'Waterval'. If you look at the line above in the newspaper snippet you provide - "Pte 7794 W Stovey" - the surname is actually "Slaney" and the casualty rolls shows the location as "nr Vryheid" on April 3 which differs from the newspaper. Some confusion there.
The 1904 "Alphabetical Index to Farms in the Transvaal" show two farms called "Wanhoop" - the real 'Wanboom"? One in the Lydenburg District, the other in the Vryheid District.