Ah, this will no doubt be very difficult. Maybe you could create some kind of SharePoint event handler that each time a form is submitted it opens the XML, extracts the base64, converts it to a text-readable format, and then submits it to a seperate database that has full text search enabled.
This is going to be a lot of work, but maybe you can accomplish what you are trying to do another way.
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge