iPads rather stink at RAW management. Apps that attempt to work around the inherent Apple limitations in RAW use are few; it's just not a pleasant work flow. Others have tried and given up (see Photosmith eg). Doing one-offs, like Express, are pretty much it.
Your request is probably better directed to Apple, although good luck with that. The iPad isn't really a good tool for photography, although as you note it could have been. Even apps that attempt to use RAW have to do kludgy workarounds, and I think the only direct import from the card is to Photos. Sigh.
You might look at Mylio. It can look at RAW on the iPad, but there are some limitations getting that synched with a desktop RAW. But basically Mylio can sync the XMP data and then you use that to transfer the adjustment info.