Terminator will soon appear in GNOME's Preferred Applications preferences if you have it installed and as such I figure we need to support -x in the same way gnome-terminal does.
What that basically means is that *anything* which occurs after -x on the command line is the command to execute and its arguments, so:
terminator -x screen -U
should cause terminator to execute screen -U. By default, most options parsers will see this as the -U being passed to terminator and screen being the argument to -x.
After looking around the docs and asking on #python, optparse seemed to be a better option than getopts, so I switched it over and implemented a callback to extend the default argument processing for -x. It wasn't quite working, so after another quick foray into #python I ended up reading this page which provided everything I needed. More than I needed, in fact, since their while loop has conditionals which affect whether or not the next arguments are added. I just want to gobble them all up and stop them from being parsed :)