Bodies consigned to mass burial and/or cremation, and those not yet found are designated as such in the cemeteries mentioned above, seems standard practice for sites operated by the Commonwealth War Graves Commission. For example, the 300 cremated after the cholera epidemic in 1943 are marked like so in Kanchanaburi. I saw the same in Bomona, New Guinea (Lieutenant Colonel William Owen was KIA in the first battle of Kokoda but as his body was never recovered his name is on the long list of missing, and so has no grave) and Kranji, Singapore. But what you are saying is that e.g. in Thailand the 'graves' are actually markers with no remains? As far as I know there was careful excavation and relocation of many thousands of remains; "the graves of those who died during the construction and maintenance of the Burma-Siam railway (except for the Americans, whose remains were repatriated) were transferred from camp burial grounds and isolated sites along the railway into three cemeteries at Chungkai and Kanchanaburi in Thailand and Thanbyuzayat in Myanmar" (CWGC site).
The original site of the main hospital was near the river next to Chungkai, to the left of the entrance. My dad also visited the other end in Thanbyuzayat - he said there's not much to see and it's in the middle of nowhere, with hazardous roads and lunatic drivers! Of course, I say this assuming many survivors of the disaster at Singapore ended up dead on the Burma-Thailand railway, including members of this regiment.