No. Scripts can't do image processing. Things like that need to be written to the RAW buffer in CHDK C code. There is an old patch (which surely won't apply cleanly any more) that added a datestamp (I think), but AFAIK it has never been part of trunk CHDK.
If someone prepares such a patch and makes it cleanly compile-time optional (i.e. doesn't increase CHDK RAM usage for those of us who don't need the feature), I'm sure it can be added.
But do note that usually this can be done outside the camera on a PC after downloading the photos, and cooking up the program or script to do that is easier than on the camera (and doesn't slow down shooting). If what you wish to write on the photos using a script can't be found in the EXIF tag (the most common watermark, date, can be found there), you can write the file name and your watermark text in a text file log from a script and use that information to batch process the files on a PC.