Exclude certain products from Magento generated XML sitemap

We're using Magento 1.7.0.2

The Magento generated XML sitemap includes all products by default, even if they are set to 'not visible'. There are several thousands of these products. I want to exclude them from the sitemap.

All of our 'not visible' products have a common string of text in the URL. I figure that this could be used by the sitemap generation script to identify them and then exclude them from the sitemap output.

I believe this should be a quite straight forward task for a PHP guru who knows Magento.

I am able to apply the required edit.... I just need someone to provide me instructions on how to apply this 'hack'.

Many thanks

Nick