I've tried that before and get an error message saying "no poster by that name." 'all' is the only thing I've ever put in a 'to' line that stays and isn't an actual FReep handle. Oh, well.
Bold the poster's name, then add a horizontal line underneath. Add a space under that, then type in a response and it will look to the eye like any other post, except it won't be addressed to anyone, in fact.