Did you attach a vertex to the dummy bones? In fact, you could just add dummy verts that arn't use in any tris.
If that doesn't work, it means mark grabs the bounds by going through the tris(Probably.), so you may have to do a dummy tri on a seperate surface, and alpha the dummy surface out.
If all that doesn't work, and he is using point in frustrum culling, I think it may be hard, perhaps impossible to work around. As there's no way to turn off per entity culling.
If that doesn't work, it means mark grabs the bounds by going through the tris(Probably.), so you may have to do a dummy tri on a seperate surface, and alpha the dummy surface out.
If all that doesn't work, and he is using point in frustrum culling, I think it may be hard, perhaps impossible to work around. As there's no way to turn off per entity culling.