Hi Francis,
from power balance principle, if the normalized insertion loss lateral offset is calculated then the normalized ORL could be calculated as well.
as a simple way is to approximate the power is to calculate the overlap areas as a power coupling coefficient.
If you are looking for one of rigorous approaches, I think the modal analysis is it.
In brief, to get the complete set of modes (radiation modes and guided modes) for both sides of the fibers then, apply the boundary conditions on the interface area (with lateral offset) to calculate the coupling coefficient. from these coupling coefficient, the reflectance and the insertion loss can be calculated. this method is in general tough (attached paper is following this procedure but different discontinuity situation)
In OptiSystem, project this attenuation could be inserted to the loss parameter after the user estimate it.
Hope this help