Uh, why not Current task? Couldn't this do? (seems that wasn't the final question 😞 )
task = Task.init() thirdparty_argparser1() #this one takes both arg1 and arg2 [...my_code...] task.set_parameters({'Args/arg1':<leave_as_is>, 'Args/arg2': None}) thirdparty_argparser2() #this one takes arg1, but not arg2