One way to do it is have a small Python (or any other scripting language really) script that performs text replacements in the Latex source file. This is much easier in Latex because it’s plain text. I don’t know of a solution that doesn’t involve writing your own code (apart from LO/Word serial letters).

Is using Latex an option? I’ve done that and it works quite nicely. You can easily populate a template e.g. using Python.