So how do you prevent yourself from getting scammed for the $1,000 hiring bonuses? You've incentivised people for up to 50-60 hours of effort to scam you. That's going to be tough to defend.
If the $1000 are coming from a larger sum being paid to them by the hiring company, I see no scam possibility (but maybe I'm just not creative enough).
There's a few vectors for potential scamming but it's going to be closely monitored anyhow. If we can't do this manually because of huge growth it'll be a great problem to have.