That's a great find!
In Windows 7 clip.exe is already there so no download is needed.
You can try this slightly simpler code:
defun( envvar2clp ( var_Name )
if(axlGetVariable(var_Name)
then system(sprintf(nil, "echo %%%s%% | clip", var_Name))
else axlUIConfirm(sprintf(nil "%s env var does not exist!", var_Name))
) ;end if
) ; end defun envvar2clp