Different taxes per line items

I have 4 types of line items: "Blank line", "taxes", "shipping" and "Discounts" (from uc_discounts)
When configuring tax rates, I can select for each rate which line item the tax should be applied.
In my situation, discounts can applied on different product taxed at different rates.
The corresponding discounts of course shall be taxed at the same rate than the corresponding product.

This does not seem possible: if I select the Discount" line item for each rate it can apply, every configured rate applies to this line item regardless the product.

How can I handle that beside using a different discount module for each discounted product?

Thanks for help