The Missouri courts have developed a nice formulation for applying the piercing the veil to corporations and LLCs. It's a three-part test: (1) A showing of control, complete domination of not only finances, policy and business practices (the alter ego test); (2) a breach of the duty of the controlling party not to use such control to perpetrate a statute or commit a dishonest and unjust act; and (3) the breach was the proximate cause of the plaintiff's injury or unjust loss. See Carpenters' District Council of Greater St. Louis v. J&J Contractors, LLC, 2009 US Dist LEXIS 36607 (April 29, 2009) quoting Mobius Management Systems, Inc. v. West Physician Search, LLC 175 SW 3d 186, 188-89 (Mo. Ct. App. 2005).
Unfortunately, there is a reference in the Mobius quote to undercapitalization that suggests undercapitalization at the time of the wrong doing may be a breach of this duty. Such a holding would upset the concept of limited liability.
Although the court doesn't identify it as such, a good example of the application of this formulation to the parent-subsidiary relationship from another jurisdiction can be found in Transition Healthcare Associates, LLC v. Tri-State Health Investors, LLC, 2009 US App. LEXIS 636 (6th Cir. January 9, 2009).