Years ago we found out that TW had a unique login type that passed the login information to the client via command line arguments. This type was and still is available in the NA client as well, and using it brings you directly to the character selection, without login form or start button. If the secondary login is enabled you do have to enter it before reaching the characters though.
Parameters:
[...] setting:"file://data/features.xml=Regular, X" /N:username /V:password /T:gamania