I don't know if this helps or not and I'm just randomly thinking here. I would think that the LODs isn't an Opsive thing. Wouldn't it be with Unity. I mean couldn't you add the LOD Group component to the weapon and assign the different LOD meshes to it.
Just out of curiously have you tried to add the LOD group component to see if it works? As far as I know there shouldn't be anything with Opsive that would interfere with it. Unless you are talking about the issue in first person view where the arms would disappear when looking at certain angles. I'm pretty sure that in the Shoot Weapon video Justin explained what to do to fix that.