ANY postdoctoral research or teaching activity qualifies for payback. There is no requirement to work in the field in which you are trained. I was trained in Molecular biology, but could work for in environmental science group and still qualify for payback waiver. These typically support students for one to a few years. Not the whole six years.
Graduate programs have more applicants than money, but try to divide it among the best applicants.
But individual faculty may themselves have research funding. A faculty thesis advisor can support you on a research assistantship. Such assistantships can be lined up in advance of entering graduate school if your record is strong and you make the preliminary contacts with prospective graduate advisors before applying.