Hi Stephen,
Is it the same material which is taxable when sold through different distribution channels ?
If not,
Option 1
I think you can maintain 0 in material masters for non taxable materials. The condition table should have both, customer and materials tax classification fields as input parameters (other the client specific required fields).
In the VK11 against the condition table you can maintain record as:
Customer Tax Class. Mate. Tax Clas. Tax Rate
1 1 10%
Option 2
You can also change the tax classification manually in sales order, in billing document tab. Altenately, with the help of ABAP you can do some enhancement to get the desired tax classification automatically in sales order while sales order creation
Hope it helps
Regards
Ankit