In Dorico is there anything that is equal to Sibelius’ system text? This would be text I can put in once and it will appear in all the layouts/parts.
My understanding is not quite yet… but in due course.
As a work round you can put almost anything in a tempo mark. Press shift-T, type what you want in the popover, then use the Properties panel (Ctrl-8) to edit the text, set the tempo for playback, suppress displaying the MM mark, etc.
If Dorico doesn’t accept the text in the popover for some reason, you can type a valid tempo indication (e.g. Allegro) and then edit it.