Hi MattE, a nice place to do what you want is in a type converter, download the sdk from the customer section a take a look to the sources of the built in typeconverters, you'll be surprised in how straight forward it is.
To elaborate a bit (As I had to think it through as well what Gus meant ): you create a typeconverter which converts string to string, so no type change, but it prefixes every varchar field's value with 'xyz'. This is then done automatically for you.