I  know that OpenSSH knows an "escape character". For example, you can
 terminate the session using "~.", even if the remote host  has  frozen
 and won't let you log out using the normal "^D".

 Well,  but  sometimes  this  doesn't  work.  I had no idea why until I
 finally had a closer look at the friendly manual:

  The escape character is only recognized at the beginning of a line.

 Aha! You have to hit return before entering "~.".  In  actuality,  the
 "escape character" is an "escape sequence": "\n~", not just "~".