It can be done through cross check. Like picking the winners from repost and then checking if they have deposited. Or picking users who are eligible by deposit and then asking their twitter handle.
For this competition in bitcointalk, I wont check twitter either.
Thanks for clarifying.
I'll probably enter the competition later in the month if the number of other participants is not too high (and if transaction fees don't go crazy again).
BTC0.2 + $20 on the top is a good deal considering there's no risk other than tx fees.