We also needed to use some custom fields with the timesheet entry and also found this difficult to do easily. We looked at using the custom action, but in the end we used a custom panel so that we could display our custom fields in the same way that the work item shows in the standard panel.
It might not be of help, but we found using the custom panel allowed us to get round the issue of only being able to access the custom fields through clicking on a day to open the entry pop up and then loosing all visibility of the custom fields once you have entered that time entry.