Can you elaborate on how ecotax is calculated and why you need to include it on the PO?
You can pass an "estimated tax" to the supplier on the PO. The simplest approach I can think of is an additional extrinsic tax field on the PO line or at the header of the PO. The difficult part is automating the tax calculation and including it in the PO total.
At the end of the day, it is the supplier's responsibility to include the tax and it really depends on how much you want to have your AP team involved in reconciling the tax. So really your best approach is to configure your invoice exception mechanism to allow eco tax through if the supplier provides the correct amount. You will still be able to report on eco-tax, but it will be done on the invoicing transaction instead of the PO transaction.
BTW, I don't know the specific of how ecotax is calculated, but if the supplier is invoicing through the Ariba Network, you could also define a custom tax type of ECO to match against the tax types you have defined in your invoice reconciliation engine. This would probably only work best if ECO tax is a fixed percentage of the cost of the goods.