It would be nice if the Working on menu had an option for 'New case' which would open a new case and automatically start tracking time against it. Since a case number doesn't exist yet it would be a little tricky, but you could do something like the following:
- Store the time when the new case entry starts (when the New Case page first loads).
- When/If the new case is opened and assigned a case#, automatically start working on it and set the Start time to the previously stored time.