The Interfaith Drug Policy Initiative has a breakdown on the various denominations' positions on a range of drug policy-related issues. Here's what they have on medical marijuana:
Medical Marijuana
"Licensed medical doctors should not be punished for recommending the use of medical marijuana to seriously ill people, and seriously ill people should not be subject to criminal sanctions for using marijuana if the patient's physician has told the patient that such use is likely to be beneficial."
Supporters include:
United Methodist Church*
Presbyterian Church (USA)
United Methodist Board of Church and Society
Episcopal Church*
Unitarian Universalist Association
Union for Reform Judaism
United Church of Christ*
Progressive National Baptist Convention
Here's the link:
http://www.idpi.us/resources/res_denominations.htm#medicalI am aware of no denomination that has taken a position opposing medical marijuana. The others just haven't dealt with it. The religious support is absolutely worth mentioning in letters, etc.