John D. Colombo (Illinois) has published Federal and State Tax Exemption Policy, Medical Debt and Healthcare for the Poor, 51 St. Louis U. L.J. 433 (2007).  Here is the abstract:

A substantial number of commentators and state legislatures have suggested that tax exemption for nonprofit hospitals should be tied exclusively to the amount of charity care the hospital provides. This paper recounts both recent state legislative activity on this front and presents policy reasons both for and against such a move, and suggests that an exclusive charity care standard may not be the best policy approach for tax exemption of nonprofit hospitals.