Complex.
You could use php for this.
str_replace($text," product ",' <a href="product.com">product</a> ');
That should replace all instances of product with a link.
You would also want to account for using Product, PRODUCT, etc. if that mattered to you.
Additionally, using " product " and " <a ..../a> " is good 'cause you would have spaces around it, rather than replacing, for example, "biproduct", or some similar word that contain the word.... it will only do full words. (You could be more creative here, too.)
Just repeat for each product/etc.
However....
Here's the tricky part--
You would need to do this to a VARIABLE, not just the "whole page".
So... find some way of storing the html for the whole page as a variable in php.... either when it's taken from the database, etc.
Are you using a forum? You could just add this into the forum's existing code for formatting posts. It would be almost identical to the process used for emoticons.
The hard part is getting the page as a variable, not replacing the words... that's easy enough.