I was going to say that exact same thing.
Well, let's put some thought into this. I would only recognize Kahi, Varderan, Venom, or Mesonak by looks alone, and I'd be most likely to recognize Mesonak if, say, I saw him walking down the road or something. First, I'd double check to see if it really was someone I knew. I'm paranoid about thinking I know someone only for it to turn out to only be someone who looks similar. Then I'd contemplate what to say, trying to think of it before they he left. Do I call him Mesonak/other cast member's username? He obviously won't recognize me. Then what do I say? Do I ask "Are you Mesonak from TTV?" How do I introduce myself? Do I refer to myself as "Willess", "Someone from your message boards", or maybe just "A fan of yours"? Do I even mention my username at all, or just go by my real name since it's IRL?
Let's say I got past all that and my own natural nervousness and started up a conversation. I'm not sure what I'd say. Although, if it was Meso, I might say "How''s life and such." Really, anything I would want to ask of a TTV cast member, I can ask here. Maybe I'd mention the latest episode or something. Perhaps I'd find out why they were where I was (assuming I'm at my home town). Maybe they're here for some LEGO event I don't know about? At the end, it'd be a nice, pleasant talk, and we'd say bye and head back home.
Now, that's operating under the assumption that I was the one doing the talking. If I introduced myself as Willess, or mentioned that that was my username, perhaps he'd have some question for me.
In summary, it'd be cool to meet one of them like that. It might lead to a long interesting conversation, or it might lead to a short one. Depends on, well, a lot of things.