I was doing the feature for you this morning, but when I get to Paypal I realize most of the people use the tax system in Paypal, this is why I stop doing the new feature.
It is useless to have the tax in my component if your Paypal is set to charge the tax.
If you want to do it without setting the tax in Paypal, you can do it in
at the end you have all the hidden field for Paypal
Line 64 you have this, the total amount for Paypal
you need to calculate the tax base on this amount.
When you have the tax amount, you need to add a new hidden field for it
This should do the job you ask