On the tax form, you can select Python Code as Tax Type.

this way you can decide if apply the tax or not depending on your conditions.

BUT the amount before taxes isn't available as local variable during the computation. neither is the quantity, so you can't just do price_unit*qty (price_unit is available as local variable).

BUT you can extend the _unit_compute method of account_tax to include what you need.

Then, you can also decide if that tax must not appear at all. To do this you can use the applicability field you find under the Special Computation tab on the tax form.

This field works only if you selected Python Code earlier as Tax Type.

To extend this to include other variables you'll have to inherit the _applicable method of account_tax.