I don't know the details precisely as we never proceeded with it, but I recall when we were looking to implement Clarity 7.5.x a couple of years ago we had exactly this discussion, and I think we discovered that using the Rate Matrix you can have a Normal Time rate, and an Overtime rate. For those resources who are billing by the day as opposed to the hour, you can simply set the overtime rate to $0. So, if you're using the Financial Management module, you should (if my memory is correct) be able to accomplish what you need.