It’s not possible to define custom tokens.
Are you already using all of the possible fields in Project Info? If not, repurposing one of the unused ones would seem to make more sense to me than hard coding text into your master pages.
Is it possible to have a field from Project Info to ne shown only on the score but not on the parts? If so, how?
For example, I added “Duration: approximately 7 minutes.” on the Lyricist field, but I want it to only appear on the score, not on the parts.