I didn't find a hosts file in the SysWOW64 folder, but I did find one in the "usual" place windows\system32\drivers\etc and I see it's NOT set to read-only. But when I try to edit it with notepad and save as "all files" rather than .txt (to avoid notepad giving it a .txt extension) it fails to save. It tells me I don't have permission (even though my user account is an administrator).
Is there some reg trick to allow me to change it? I've got a bunch of mappings I want to put in there.
Fairly huge. Larger than Windows need to protect it
The answer was annoyingly dumb. Just copy the file to another location, edit it there, and then copy it back. Then you're only presented with a typical administrator privilege prompt, to which you can click "continue" and it's done.