This would be a nice feature :D
A time matrix (weekly) were you could put the % of change for the billable rate assigned to a project.
This feature applies to the cases where you have more than one rate depending on the hour. For example, tasks done at night could have a 50% increase rate.
Additionally, you could put a tag to the different time ranges (NBH/normal business hours, ONBH/out of normal business hours)