The problem with this is that because lying or omitting the truth is so common, you end up communicating more than you may intend to when you are honest and forthright. For example, it is impossible to comment about being attracted to someone without also implicitly communicating that you want to either go on a date with or sleep with them in most cultures. Or if you do the opposite and tell them you find them hideous, it is generally assumed that you must have told them this because you wanted to cause them distress.

I think this whole thing could maybe work, though, if you take great care to be very aware of what you are indirectly communicating and head off those assumptions. In many cases, this may simply require that you first qualify things with "Don't take this the wrong way, since I'm trying this Radical Honesty thing, but..." or "Can I be blunt with you? [yes] I think..." In other cases, this could be more specifically addressing the feelings you expect to elicit, "I know this is going to make you feel bad, but I think it will be better for you to hear this from me now, ..."