Get the sources for knocker from http://gehennom.org/doc/knock/knocker.c. Modify the list of magical ports and lengths in the source. Compile. (gcc -o knocker knocker.c.) Use with: knocker [-v] open/close <host>.
Note that closing the port afterwards isn't absolutely necessary, since SSH connections are only allowed for the 30 seconds (or whatever you configured the delay for) immediately following the knock. Closing merely servers to clean up the source address from the lists the recent extension keeps.