[ic] multi SalesTax by quantity?

[NAMI Solutions]

I'm currently setting up sales tax calculations based on tax_categories.  
I'm using the SalesTax "multi" directive and everything is fine, except that 
for certain products the calculation that I was given needs to be based on 
quantity.  So if the customer orders 5 of these items it needs to use a 
fixed tax * 5 to give the correct result.  Is there a recommended way of 
doing this within Interchange? 

Thanks,
Mike