Its not a perfect solution, but you could rename the assignment something that would not be obvious to students (e.g., Bonus points) without an explanation of what it is until some deadline has passed and you have entered the extra credit.
I have a similar situation where I want to give students extra credit for reading the syllabus, but because my class is 180 students, I don't want to receive even 20 emails notifying me that they found the extra credit. I'm wondering if there is a way to create an assignment in canvas where the students would enter the extra credit if they find it? That would eliminate the need for me to be involved.