If I was creating a sig program that was fair, I would think the following would be good ideas:
Allow any member, regardless of post count/join date.
Make the payments either weekly, or as you said, have a portal where they can be paid out whenever the person wants.
MY MAJOR IDEA WOULD BE THIS:
People make tons of spammy posts, and it can be hard to go through each accounts posts, to ensure that they are quality posts. Hire someone to do that for you. I WOULD LOVE THAT JOB...... I ave the free time to go through posts to make sure they are not just random bumps to increase the post counts. Please keep that in mind.... it helps you pay for quality, not spam, and it gives me a job to do, since I am online all day for 16+ hours a day anyways.
The reason why old members are preferred is that the signature is applied to all their posts.
A good non spamming member will make 20-80 posts per month.
So , if a company hires you for a month , they will probably get 40 posts out of you , along with the signature being on 68 old ones.
They would get around 110 posts with their signature.
Now consider if a company hires me. I too would make around 40 posts , with the signature being on 310 old ones.
Instead of getting 110 posts , they would get 350 posts with the signature.
Secondly , spammy posts may be bad to users , but are not always bad for the companies.
A person on primedice advertising who gets paid 0.8 BTC has to post 4000 posts , the average of a hundred regular users.
While their posts may be annoying to most people , the signature is still plastered everywhere.
Nobody on the forums has not heard about primedice.
Back on topic :
Vineyard , I have sent you a private message.
Please read it and respond.