AR Tax Setup

I am busy doing the AR setup for a company in  Cameroon (on 11i) and I am looking for the best way to do the AR Tax Setup.

There are 2 taxes to show on the invoice.  I have set this up with Tax Groups.  The VAT rate is fixed for the entire country.  The difficulty is with the PRE tax.  Company VAT registration numbers are only valid for a certain period.  The PRE tax rate varies depending this.  Once the end date is reached, the PRE tax rate we put on the invoice has to change.

At the moment we are looking at putting the Tax Registration end date as a flexfield on the Bill-To site and using custom code to determine the PRE tax rate.