Hello @Xacto - welcome to the forum.
As far as I know, there is no way to use an AppleScript or Terminal command to ‘force’ a file to be online-only or local-only.
However, if you try to ‘read’ the file in Terminal, it will cause Dropbox to download the file locally if it is online only.
So, for example, if you wanted to make ~/Dropbox/Some File Here.pdf local, you could do something like:
cat ~/Dropbox/Some\ File\ Here.pdf >/dev/null
which will force it to download.
I am not sure this actually helps, but unfortunately the Dropbox app for Mac is not automation friendly at all and never has been. Clearly they don’t think it’s important, unfortunately.