Can this be done? At the moment, the only way I can think of doing it is to duplicate my current Style, and set one to use serif fonts and the other to use sans-serif, then users can choose whichever they prefer. However, this is a very clunky way of doing it, and I'm inevitably going to have some people moaning that I've chosen the wrong fonts!
Even just giving people the choice between serif and san-serif would be enough. I would be happy to pay for such an Add-on, if it wasn't too much.