In WindowMaker, it is either the default, or an option, that when docked app
launcher's are invoked with a middle-click, they are invoked with the
clipboard contents as the argument.

I would love something like this in Xfce. Even allow the user to configure the
cmd to invoke w/ the middle-button.

Perhaps even there could be some standard variables that could be specified on
launcher command lines that would be expanded at the time of launch.

For example, there might be a "clipboard contents" variable so, I could use a
command line like this:

MozillaFirebird $(clipboard)

and that would invoke MozillaFirebird with the contents of the clipboard as the
first argument.