If you do need to mirror it after insertion, I've found you need to use a rotation matrix rather than a mirror whatever.
edit; Mirror it about a picked point using the x-axis as a vector and rotate about that same point using the following math.
Below is the guts part of a mirror function protected override bool WorldDraw(Autodesk.AutoCAD.GraphicsInterface.WorldDraw draw)
{
if (mir)
{
Vector3d V = Cs.Xaxis;
Plane P = new Plane(M1, V);
transMat = Matrix3d.Mirroring(P);
NewAng = Math.PI * 0.5;
mir = false;
}
else
transMat = Matrix3d.Rotation((NewAng - Ang)*2, Pn.Normal, M1);
Ang = NewAng;
foreach (Entity draggingEntity in m_entitiesToMirror)
{
draggingEntity.TransformBy(transMat);
draw.Geometry.Draw(draggingEntity);
}
return true;
} //End WorldDraw