Read string from file in Keynote

I don't know of a way to do that, BUT, if you make a "template" file, you should be able to open it's internal .apxl file in TextEditor and do a find/replace.

A few things to get you set for this, is to go into Keynote's Preferences and, under "General" and in the "Saving" section, select "Save new documents as packages". Going forward, all your new documents can be opened like packages from the older version of Keynote.

Once you have that set, create your "template" document, save it, then right-click (control-click, two-finger-tap) it in the finder and choose "Show Package Contents". In there, you'll see an "index.apxl.zip" file. Double click it to unzip and you'll have index.apxl. Put the .zip file in the trash and empty it (Keynote can read either and you want to make sure the edits you're making are being used when yu open the doc to test).

Now, open that apxl file in TextEdit and do a search for your dummy text. From this point, it's just a matter of replacing that text with the new text, saving the text file, then opening the document in Keynote and checking it out. There may be minor formatting issues, but that's a LOT better than having to flip between slides from start to finish.

Create a new empty document using the same theme. View that document in the Finder to see if you can open the package. If you can, then open the original document, copy the slides, and paste them into this copy. When you save the copy, it should be as a package.