Custom field per work-log

The custom field are very nice, but to have the time tracking system address all of our needs, we would need to have custom fields per work log. Some example categories we would want to track:

• date work was done (the date you write down your hours isn't always the date you did the work)
• intended billing rate (what were we billing when we did the work)
• actual billed rate (how much were we actually able to put on the invoice)
• work type (development, testing, documentation, etc...)

If we had the ability to add custom fields per work entry, we could add these fields as necessary, then build our own reports to produce invoices, burn rates for the month, utilization, etc...

Hopefully this isn't that great an extension.