Tax on global purchase agreement lines

The great benefit of Global Purchase agreement lines is the ability to upload large number of lines with no need for items. The big question I have is about tax. Tax codes can be assigned to an item. But there is no posibility to assign a tax code to an global purchase agreement line. How do we calculate the tax on a requisition and purchase order line related to a agreement line with no item? Now we have an customization to do so, but I rather not have this customization.