Mirrored parts are easy to create. And it's just as easy to create them independent of any assembly.
The procedure is similar to the Master model merge technique
as far as setting up the assembly. The point is to assemble the components without
assembly references and make sure to pick a datum plane from the reference part
as the mirror plane.
The final check is always to check the model info for the mirrored part to make sure that there are no assemblies referenced.