you could do this by creating a database table to hold the tax rate, and storing the tax rate in a session variable.
on the cart page, create a recordset to lookup the tax rate in the database, then add a set session value behavior.
on the server behaviors panel, click the plus button and select eCart -> General -> Set Session Value.
set the trigger to:
If Session Variable is not defined
set the name to TaxRate
for the value, click the lightning bolt, and select the tax rate column from the recordset.
you may need to rearrange the code so that the set session variable behavior is after the recordset.
then in the tax rule, you can refer to the tax rate using $_SESSION['TaxRate']