I'm not sure which new system you are referencing, however:

This is one reason I think that hourly reimbursement should go to core code maintenance and nothing else. Feature development could be community funded, bounty driven, proposal defined, or board managed.

Under the hour reimbursement system -- the one that exists right now -- I would think that the developer would record hours for the months they work on the project and be reimbursed for those hours at those month's GRC price when the feature is implemented. They would be reimbursed for the time they spent on the project.