How to indicate the original clef in Musescore 4?

I do it in MU3 with the camera tool. Set up your score using the original clefs, then use the camera to make SVGs of them with transparent background. Save them somewhere, then change the score to modern clefs. Use a horizontal frame to indent the first system as needed and then add your graphics at the start. It is common and clearer to leave a gap between the graphic and the staff.

Unfortunately MU4 doesn’t have the camera tool but this forum describes other methods using system tools such as screenshots.

You can fake the staff lines and clef with special symbols.
To be able to insert special symbols in MS4 you have to insert any text and double click on it! Then in the properties you can invoke the "special symbols" window and drag the stafflines onto an empty space in the bar.
Now disable automatic placement for it in the properties!! And drag it into your desired place. As the symbol is quite narrow I had to insert a 2nd copy of it next to it. (again disable automatic placement!)
This way you can also insert a clef symbol.

(Instead of symbols you could also fake the 5 lines with 5 horizontal lines from the lines palette (if you would like to have more control)
Feature Request 1: Allow invoking "special symbols"-window directly without "text-edit-workaround"
Feature Request 2: If Musescore would allow to hide system brackets individually then it would also be possible achieve this with separated first bar.