Sweeping Tox in ngspice
If you look at section 1.1 in the BSIM4 manual, there is a description of the oxide model.  You just cannot "sweep" the model and get anything close to real behavior, but DTOX can be swept.  

At some point, you'll start to tunnel when the oxide becomes thin.  Also, depending on the process, you'll see "gate leakage" due to the dirty etch from the drain terminal to the gate.  This looks like gate leakage, but you can mitigate it by changing your etch and moving out your contacts.