Since the same item and customer combination have different requirements at different times I believe your only choice is to make a decision at Sales Order Entry.
In other words you could use advanced pricing in conjuction with the PC1 field in sales order entry. Create two advanced pricing adjustments one that will give a 100% discount off the base price when you enter some code (ex NT - 'No Tax') or (UT - 'Use Tax') in the PC1 field at order entry. And a second adjustment that when you enter the 'UT' code activates a non-taxable type 6 adjustment that will calculate your tax rate on top of the base price and include it as a line on the invoice.
I'm no tax expert but if you don't need to charge the use tax to your client but only one to account for it in your books you could use a GL accrual adjustment.