Hrm. The ixus90 appears to have 12-bit depth for it's RAWs. Don't most of the other P&Ss have a 10-bit depth? It still doesn't explain why when I'm reading the RAW buffer the data is incorrect.
Raw Size = 0xEC04F0 (from firmware)
Width = 0xE88 (from firmware)
Height = Width - 0x3B4 (from firmware)
Raw Size = Width * Height * 12-bit / 8-bit (bytes)
[edit]
DNG4PS assumes 10-bit depth, thus if I only save the equivalent of the Width*Height*10-bit DNG4PS will recognise the file but of course the RAW file I end up is essentially noise.