The ability to change the order in which tax providers are executed. An OrderBy field and some changes to the TaxGatewayDataSource to order it by that field should be sufficient. I had to change the TaxGatewayId of the records (not advised ofcourse) to make it work because i wanted to apply a tax rule on the taxes added by my custom tax provider (tax on tax).
AbleCommerceTax is the first Tax Provider applied to the basket, since it is the first row in the ac_TaxGateways table (created when you install AbleCommerce).