I'm wading into this a bit late so forgive me if I overlook anything. It seems that the core of the debate here is whether Mr. Kostric helped, hurt, or had no effect on RKBA. In order to find the answer to this question, I think we need to examine the facts of what happened. Mr. Kostric broke no laws and was never even particularly close to President Obama, he was apparently left unmolested by the Secret Service, he made no overt threats (his sign and shirt could possibly be construed as one but I think that's stretching things a bit), and he managed to remain calm and controlled during what I consider to be a grilling by an obviously irate Chris Mathews. So, what messages were sent by these events? I see two, the first and most obvious is that firearms and the people who carry them can be present without leading to violence. In watching the interview with Mathews, it appears to me that his was the main message that Mr. Kostric was attempting to send and, because there was no violence, it would appear that this message was sent successfully. The second message that was sent was a bit more subtle, given the recent acts of intimidation and outright violence carried out against people who oppose the government at town hall meetings, I think Mr. Kostric sent a message telling those who would carry out such acts that not everyone is as easy a target as they might think. In essence, Mr. Kostric's gun served as a deterrent.
So, are these messages helpful, harmful, or indifferent to RKBA? To answer this question, one must look at this incident from the standpoint of the casual observer. Obviously, Mr. Kostric's actions aren't going to change the minds of those on opposed ends of the spectrum regarding this issue. Someone who is frightened by Mr. Kostric's actions is acting on the basis of emotion rather than logic. An outright fear of firearms is, IMHO, irrational and thusly no logical argument can change it. Such people are arleady unlikely to be supportive of our cause and thusly Mr. Kostric's actions regarding them make no difference. Likewise, people on our end of things are unlikely to be made into anti's by Mr. Kostric. They will argue that even if Mr. Kostric's actions were ill-advised, counter-productive, or inappropriate, the actions of one person do not justify an infringement upon the rights of everyone else. Therefore, the only people who can really be impacted by Mr. Kostric's actions are the fence-sitters: those who have no opinion or who are still formulating one. To these people, I think the fact that Mr. Kostric did what he did without an act of violence occuring may mildly influence their opinions.
What I think will probably influence opinions more than anything is the way the Mr. Kostric handled himself during the Mathews interview. The fact that the supposed "gun nut" remained calm and rational while the supposedly impartial journalist lost his temper doesn't really jive with the steriotype of gun owners as a bunch of looney, anarchist, Rambo-wannabes that the anti's have been promoting for years. Through their own attempts to spin and outright bias, the slanted journalists (namely Mathews) as MSNBC have actually helped our cause by discrediting their own. That, I think, is more powerful that the event itself.
As to the second message that Kostric sent, I kind of doubt that most people outside of government will pick up on it. However, for those that do, I think the message is quite powerful: You don't have to just roll over and take it, you have both a means and right to defend your freedom.