The reason calling LoadOpenPositions doesn't work is because it is a static method which doesn't modify anything about the PositionManager instance. It justs reads the data from the file and returns it as a PositionXml object. When RightEdge uses this method internally, it then goes through a lot of logic afterwards to set up the open positions.
It sounds like you may be trying to do a walk-forward optimization. In this thread
I describe how you could do so.
Allowing you to set the open positions before starting an optimization run would make writing a walk-forward optimization plugin a lot easier. It's something we'd like to support in the future, but it won't necessarily be any time soon.