Is there a way to pick a default character set so that it doesn’t ask me every time I open a Writer document from the desktop?
It means, it’s not a Writer document, but e.g. an HTML, or another external file format.
You may create a macro to open this files from inside LibreOffice. The macro can fill the needed fields for the filter. But only you can decide, wich is the (most likely) charset.
.
On my system you find old IBM437 charset in dbase-Archives, a lot of modern utf8 with and without BOM (and one utf16-text every 3 month) and a diverse mixture of Latin1/Latin 15 (with €) texts.
.
Asking me instead of importing csv-files with errors was one reason to change from Excel to OpenOffice for work long time ago.