Scripting- Retrieving Tax Schedules

I am trying to create a sales order based on an existing sales order record.

var soScheduled = nlapiCopyRecord('salesorder', salesorderID);

The items in the sales order will need to change and so will some additional fields on the sales order.  My question is, how do I load a tax schedule associated with an item?  I see the id of the tax schedule that is associated with the item, but tax schedules are not scriptable records according to the documentation. 

var state = getCustomerState(...);
var taxSchedule = nlapiLookupField('item', itemId, 'taxschedule');

I need to then get the schedule to see if the state is taxable for the item.