If you are really serious on your giveaway then you should require them to apply here or fill up a registration form so that you can track them and check whether they are still wearing signature or not. Because this campaign will be easily abused in case you REALLY WON the jackpot. Anyone can wear the signature right after you announce that you won the jackpot.
And re-echoing statement above. Most of the user will be recently woke and participate here will be hack or bought account so beware.
Adding another important point, OP should also consider displaying the participant sheet to the public here in this thread to avoid misunderstanding at the end of the bounty.
But how can you verify that people are using the signature all the time?
It could be manipulated by put the signature immediately after they find out your ticket are won.
This is interesting and a bit complicated at once.
-snip-
He can close the participation a day before the lottery drawing day. It's simple if he is really into it.