trying to do square notation in Dorico

It works fine from Finale.

Maybe my Finale is too old – 2010 … Funny clef placement, by the way!

Indeed, but I can’t imagine you’d hang onto that treble clef anyway.

Just tried again today.
Importing a modified music.xml into an existing Dorico project does not work.
But opening a (modified) music.xml file with Dorico will import it with the right number of staff lines.

Yes, you’re right: it will only work as a new document. Sorry.