I've tried photogene, snapseed and photoraw in trying to find a reliable way of previewing DNG files from my Ricoh GRDIII and GXR with limited succes.
The photogene and snapseed only read the jpegs.
However Photoraw works for the GRDIII and GXR A12-M.
The lite version of photoraw is free - so at least you can see if it works with your camera before paying $9.99 for the full version ( but beware the lite version is so crippled it locks you to the first camera you try it with and has no file export).
As Gigabytes are so expensive on the iPad, and I've only got the 32GB one, my plan is to keep spare SD cards for the camera, and only using the ipad for previewing or quick editing jpegs.