keep it a secret

No, that would be demotivating for the previous participants. It just doesn't have to appear so prominent. Maybe a "BTW" at the end of the article...
Instead, you could open a kind of "Hall of Fame" in which the weekly victories are recorded. Once a month?

new people might get tagged

"New" people aren't tagged anyway, only those who have won and have the game in mind anyway.
Besides, tagging is a tricky thing. Some people like it, some people need it, some people hate it. I hate it and am only here a) because I look at my feed b) because I'm interested and have no other ambitions c) for your sake.

watching my feed for updates was too much effort.

Poor, poor Steemit, where have you gone?