I'm not too familiar with the DNG format, but if a DNG file could be as simple as [header][2304x1296x2 bytes][footer] then it might be possible with the current hardware.

But without a filesystem, would it be useful to have DNG files stored on the SD card? Wouldn't you need special software to read off the "packed" DNG data anyway? And if you need special software anyway, couldn't the special software convert the existing raw image data to the DNG format, instead of requiring the hardware to do it?