There are a few apps around, although I have yet to find the perfect fit.
Sortshots, http://www.sortshots.com/, is one option. You load the photos from Lightroom onto the iPad with the Itunes file sharing feature, and you can then edit metadata on the ipad offline. The results can be sent back to Lightroom by copying the photos out of the iTunes file sharing feature.
The drawback of this workflow is mainly three things:
1) Cumbersome to manually drag and drop photos between iTunes to sync
2) Not good workflow to get metadata back to e.g. Lightroom, as you need to overwrite the old photos with the new edited ones from iPad (if I understood it correctly)
3) If you want the photos to appear in the photos.app, you need to have the same photos twice on the iPad, eating double disk space.
To Rebuild the Photos.app Thumbnails:
Backup your library before following these instructions.
Delete the old thumbnails and various parts from the library:
Quit the Photos app.
Open Finder, and go to the Photos.app Library, on my system it is called Photos Library and is found in the Pictures folder.
Now right click the library and click Show Package Contents.
Next delete the folders called Previews and Thumbnails.
Then delete the files called ImageProxies.apdb, ImageProxies.apdb-wal, Library.apdb and Library.apdb-wal from the database folder.
Lastly delete the folders RKCloudResource, RKImageProxyState, RKMaster and RKVersion from the recovery folder.
Now rebuild the library
Open the Photos app while pressing ⌥+⌘. It will ask you to if you want to repair the library, click Repair. It will do a repair, then your photos will go through a reprocess/reimport process. Once finished the thumbnails will start rebuilding.
P.S. I tried this on my own Photos library, and it worked.
Best Answer
In OS X 10.10's Photos application, location information is read-only.
Photos reads GPS coordinates from image files during import and displays the location in the Info window. It can also show photos belonging to a collection or moment on a map. The "Photos Help" puts it this way: "If your camera has GPS capability (as iPhone does), or you’ve added GPS information to your photos, Photos can automatically display your photos on a map."
Unfortunately location information is also read-only by the way of Apple Script. I develop the photo geocoding software HoudahGeo. When used with iPhoto or Aperture, this allows for a very elegant workflow:
The last step of the workflow uses Apple Script to update the library to reflect the location information added to the files. This is not currently possible with the new Photos application.
Until Apple adds the option to change location information via Apple Script, we have to resort to a less elegant workflow.
This workflow does have its advantages. The process of updating the iPhoto or Aperture library for thousands of photos can be rather slow. Adding the GPS information before import is faster.
The above workflow also ensures that GPS information is stored as EXIF/XMP tags within the image files. Just the same as if you had used a GPS camera. The location information will thus always stay with the images.
Update for macOS Sierra:
With Apple Photos 2.0 and HoudahGeo 5.1 running on Sierra it again possible to notify Photos of changes made to location information. I.e. HoudahGeo will geotag orginal image files with EXIF/XMP tags and then pass on location information to the Photos app.