I would suggest using a different approach. Create a layout with fields that are editable, then place these fields as merge fields on your print layout. http://www.filemaker.com/help/html/create_layout.9.23.html
I take it that's an AppleScript that launches FM and then enters layout mode.
My suggestion would be to rethink your solution to leverage the functionality within FM. When the user it modifying the layout, then that becomes the next layout used for the next "letter". Some examples:
Create a "Data" table and layout (Named as you need it), this is where various pieces of data are inputted (Name First, Name Last, Name Prefix, various address fields, phone, email, etc.). Then create standard layouts as letter templates where the data is merged into the letter. You could have fixed layouts for letters that don't change and different layouts for letters that are pretty standard but have the ability for the user to input a "custom" paragraph in the Data Entry layout.
Another way would be to have a data entry layout and then a letter layout with standard openings and closings. You then select a script to populate a "Body" field with preset text. Once the body was set by script you could then make that available for changes by the user.
Just a couple of thoughts, directly editing the layout for each time the database is used might not be the best road to go down. I do employment letters and contracts for my organization which currently includes 22 different standard documents and one that the Compensation Analyst can use which produces a generic format (Header, opening, signature section, and footer) - she then inputs the "body" in a special tab on the data input layout. She can then Preview and Print as needed.