That can be okay, but this would keep allowing users to publish the song immediately, and that would make it really hard for another user from the same nation to change the song, because that user would feel like he is complaining about the entry with the other user.
I would rather set two completely different periods in the TISC schedule: one for National Jury Registration and after this window close, there will be one window for entries selection.
Right now we have:
(are the two bolded events the same? If so, you can use the same name for both)
My idea would be:
Dates are random and can be changed, but the point is no one can post the song before the national jury registration close. This would have two positive effects:
1) Songs would be published more or less in the same period, avoiding one song being published in February and one in April (this could be an advantage for the one published in February, because users would have more time to listen it).
2) Users from a nation already represented would be more motivated to join because they will take part in the national entry selection.
To give users who join first still a little advantage, you can say that if there is a jury with 2 or more users and it is really impossible to find an agreement to which song will be selected, the user who first registered in the contest will win the "tie-breaker".
Also, users who join the contest between March 21st and April 10th might still take part in the contest and in the voting process, but will have no word in deciding the national entry.