truss rod at limit on LTD viper104 bass- next step?

My LTD viper-104 bass has too much relief. It's about 1mm or .050in at the 9th fret when the string is held at the 1st and 21st fret. The truss rod works fine - I can fully loosen it and tighten it and it clearly makes a difference. The problem is the rod reaches it's limit before I can get the relief to a reasonable amount.
(Note I did remove the strings and put back pressure on the neck overnight in my tightening process.)

The truss rod is dual action with a captive nut. I'm not sure of the actual dual-action design but it looks like it could be like the image attached. Does anyone know the design of the LTD truss rods?

I'm wondering if I can drill out the end of the truss rod that holds the nut captive? Then I could remove the nut and place washers under it. Of course it no longer be dual-action after that.