I would suggest you create your own 'Manager' which controls all the "business logic" behind weather something can be added to cart or not. You just need to implement a few hooks in a pseudo module. I didn't think that people would want to change the error messages so that would require a hack but I'll be sure to update in UC2.0 versions to allow this.
Look at the 'uc_stocklevels.module' file for an example of a Manager. Also someone has writen a Manager which sits on top of the uc_stock module which might also be instructive.
good luck mate