Perhaps if the bookmarklet opened a new url containing the text as a query parameter, and the url had a weird protocol instead of https:// — say it was snippy:// … and then you created a protocol handler on your machine for the “snippy” protocol …. And it saved the relevant query param to a file.

I can’t remember if I’ve done this before. It seems like something I’ve done.

I've done that too. I ended up just using a local webserver with some Python cgi scripts instead though. I found it more flexible and easier to remember how things worked that way.

Yes, the other easy way I have used multiple times is, bookmarklet simply sends the text via a POST request to Google Apps Script, which simply appends it to a sheet or doc.