Some thoughts:
If you put the rate in the campaign's topic title, you'll get many applications.
According to Overview of Bitcointalk Signature-Ad Campaigns, these are the highest pay rates for lower ranks. Once word gets out, you can expect many applications including alt-accounts. I know from experience that checking posts from Members is a lot of work, as most of them aren't worth it. Jr. Members will be even worse.
You can make it less work by using Merit for selection first. If you happen to find a rare Jr. Member with good posts but no Merit, please report them here so we can Merit them.
You have the rare opportunity to mean something for lower ranks, it would be really nice to see that used to reward the rare good users. Just imagine when a Member wearing Stake signature means he's a great poster!