Basically, since I don't want to spam my friends with "Toaster is playing Fallout New Vegas" repeatedly when testing, does anyone know if there's a way to quiet-launch games on Steam?

You could just go into offline mode.

No disrespect for piggybacking off the OP's thread but what about a way to disable the startup splash screens so that it goes directly to the main menu, I would like to speed up testing process as much as possible.

They used to be ini entries, IIRC.

Just change your status to Offline so your friends see you as offline. It shouldnt tell them anymore. You can still earn achievements and all. And if they send you a message you still get it. (I think)