If you aren't using Color Labels, Star Ratings, or tags yet, I think utilizing one of these will allow you to speed up this process a bit. Here are the steps I suggest if you want to use the color label (tags work slightly differently so I can explain if you need):
First, choose a color label to represent "Has GPS"
Then, choose "Select Photos with GPS" as you've been doing, and mark those files with a color label.
Next, in the filters above the Contact Sheet, click the color that you've chosen, and all items which were given that label will be hidden. You'll be left with only images that do not have GPS coordinates.
Apply coordinates to the next batch, then apply your color label (definitely make a note of the keyboard shortcut for setting the color you want if you don't already have that memorized), and then hit Command+/ to refresh the Contact Sheet. The color labeled images will disappear and you'll again have only the images without GPS coordinates.
As far as Copying and pasting. The easiest way I could think of was to do the following:
Go to Edit -> Settings -> Set Info Text
In the text box of the window that appears, enter the following variables, each on their own line (if you're doing this often, you may want to put it at the top of this text box so that it is the first info that you see):
{latitude}
{longitude}
Then, accept the change. Now, when you open the Preview window, in the info panel on the top right, you'll see the GPS coordinates from your image. Select the text and hit Command+C to copy. Then in the "Set GPS coordinates" window, click "From Clipboard" and it should fill in your coordinates. It would probably make sense if we had a "Copy" button in the dialog itself, but this is the best method I could think of in the current state of the app.
Hope that helps!
