If you have DITA maps you should look into XSLT. That will be much easier than scripting. You could even load the map in FrameMaker XML view and do a search and replace there if the pattern you are replacing is easy enough. In the XML view you can also run an XSLT.
I have to confess that since I put this question in writing I remembered that I had "GrepWin" on my pc and when I tried it it was able to find and replace the text in all the dita files in the present folder and sub-folders in one go. Now I can port it to my local drive and operate without any trouble... as yet.
ExtendScript has a File object and Regular Expressions so you could do this within FrameMaker as well if it is more convenient.
I do remember that you and I had a conversation where you said we could make a a script for this function. I would still like to learn how to do that. If it is simple enough would you be willing to post it here?