If I replace the oil tank on my DT360a and disconnect the oil line to the oil pump at the tank, do I still need to prime the pump again or, given the line is still pretty much full of oil, over say a week, will gravity fill whatever air/space is in the line?

Depends on how lucky you are.  If you are a lucky person, yes the bubble in the line will rise out to the top of the tank.  For me and the devil bike, I would never dare take the risk.  Bleeding the pump only takes a few minutes.