Actually, I found two tax court cases, but they seem unfavorable to your position.
In Pavia v. Commissioner, T.C. Memo. 2008-270, it was found that food stamps and Medicaid benefits were to be considered support not supplied by the parent(s).
In Rasco v. Commissioner, T.C. Memo 1999-169, it was found that welfare (for an adult) and food stamps (both for an adult dependent, and for children) was to be considered support supplied by the individual (adult or child).
In both instances, the taxpayer who claimed the dependents kept the exemption, as, even including the welfare payments, the taxpayer supplied more than half of the support.
The regulations at 26 CFR 1.152-1 specify that Social Security benefits are included in "support", but it's not clear to whom the support should be attributed, so it's not particularly relevant to your case.
I'll see if I can locate Revenue Rulings
on point later today (Sunday), but my topic index is not the best.
Edited by Arthur Rubin on 6/6/2010 at 10:20 AM EST