How to fill a field in all symbols in a schematic with a "formula" (ie. a default value)

The "Field Name Templates** are ecactly that. It’s a template for adding fields to symbols which do not have those fields yet.

If you want those fields to filled in in advance , the most logical option is to add those fields + their values to the symbols in your library.

It also reminds me of this recent thread:

Another possibility is to not do it at all. It looks to be redundant information, and you can probably create it by some customized BOM script.

It also reeks a bit like the wish of a database library.