I've called around to a few of my schools. The people who answer the phone at the financial aid office, for the most part, didn't know what to do with my situation - They've mostly had to escalate my question up the food chain in order to research the issue. Evidently children of single parents getting accepted to medical school is a rare event, because this question apparently doesn't come up often.
So far, it's been a mixed bag, with a couple places asking for a degree of documentation that I think is unreasonable. For instance, one place said "Just write NA for his name and 0 for the dollar values, that's fine," only to email me the next day saying, "Actually, no, we need you to write us a letter explaining your family circumstances and have a third-party professional such as a lawyer, clergyperson, guidance counselor, or social worker write us a second letter attesting to your situation." Eek. I don't have anyone that's known me my whole life, much less a professional third party. Called them back and explained that I don't have anyone who can function in that capacity, and asked if I could just submit my father-less birth certificate. They're looking into it for me and I'm stuck waiting to hear back. I'm getting the impression that proving an absence is going to be tougher than I'd like.
To be fair, some schools have been VERY considerate of my situation, and I am so, so grateful to them for not making me feel like a second-class citizen for coming from a fatherless household! In fact, one school was like, "Are you kidding? Of COURSE it's okay to just send us your mother's tax forms! What school's giving you a hard time about it?? That's just crazy." What a sweet woman. I legit wanted to hug her.
(Before anyone starts defending why it's important for schools to have a clearly-defined, "fair" appeal process, let me just say that I definitely understand where the schools are coming from. I'm just blowing off steam because the process, in my opinion, imposes some unrealistic expectations on the applicant. Regardless, I'm sure everything will be just fine. Breathe in, breathe out, repeat as necessary.)