Nailed it.
There is no definition. It's a hollow title used to justify any personnel decision. It means nothing, and frankly...with the results on the field, I wouldn't mind adding some wrong kind of guys.
Let me just point out that not one coach, dc.com writer, or top brass has said that McClain is RKG material. I haven't really even seen any optimist on this blog trying to tout him as an RKG. The idea behind this move is to find out if he can be that...if he can't, they simply release him and lose nothing for their trouble. Given the loss of Sean Lee, this was a darn-near necessary chance they had to take. Obviously, McClain has his work cut out both mentally and physically to make the final 53, but this very well could be his last chance at having a productive career in the NFL.
As far as there being no definition to RKG, I disagree. Perhaps people has defined it differently, but at the end of the day it boils down to a few things, none of which includes boyscout, choirboy, or country club member. It simply means you have a passion for the sport you play, you have a willingness to put in the work both on and off the field to be your best come gameday, and that for the betterment of the team, you support your fellow teammates in that ambition.
As far as McClain is concerned, he has alot to prove...but I wouldn't say he's a lost cause; especially if he can return to that player he was coming out of college. With the right coaching, the right team atmosphere, and the right support system, he could turn out to be a great addition to this team. But it is all up to him to prove he can be that. As I said before in this thread, he's not a guy that's going to top "most likely to succeed" list; he is clearly a longshot. But if he does turn out to be a solid contributor for 50% of the defensive snaps in this next regular season, he more than warrants a 6th round pick.