The only idea that comes to mind is to create a graphic of the diagonal text and bring that file into Finale using the graphic tool.

You might want to look into a 3rd party app to watermark a PDF of the file. That way you could control the transparency. Anything you do in Finale would overlay the score.

Rotated text can be created in two ways:

1) Use the Smart Shape Tool.

Create a custom line (in the sub-tool Custom Line Tool).

Give the line zero thickness, and add your text on the line (look in the bottom of the Smart Line Designer).

2) Create the text as a custom arrowhead.

The custom arrowhead will be created in the Shape Designer.

You can use your custom arrowhead in two ways:

a. As an arrowhead on a Smart Line - instead of a text along the line as described in 1)

b. As an arrowhead on a line in the Shape Designer’s sub-tool Line Tool.

This means that you can use rotated text as part of a shape created in the Shape Designer.

You can use a shape in several tools, such as (e. g.)

- the Expression Tool (= a so called shape expression)

- the Articulation Tool (= a so called shape articulation)

- the Clef Tool (= as a custom clef symbol)

Here’s my favorite new-found way to do this in bulk on Mac:
https://www.techjunkie.com/how-to-watermark-pdfs-mac/

There’s a YouTube video of another method for PC too:
https://youtu.be/QmK3JqbKXlU

I know it’s is an old post but I hope this helps anyone who ends up here.

Both methods are free, by the way.