The motd windows steam login was removed a while back (or never existed?) because servers were using them to issue steam://connect/IP
ORT/ protocol which would redirect players to another server without asking. These protocols were removed from interacting with windows and other applications also so you couldn't force open teamspeak with the protocol and make the player connect to it.
I don't think you could ever force open a webpage under a player's steam overlay. The steam overlay is pretty independent from the games themselves.