One is considered a federal employee if 1) one works in a position for the federal government or 2) derives an income that comes from using federal property (i.e. using federal land for logging) or 3) is paid by their employer who maintains a contract with the federal government (thus one is included under sub-contracting laws). Federal employees were the entire reason the income tax was invented. In other words, even a trust would still be liable for income taxes under one of these three positions. The only work-around for this type of employee is to utilize the lawful money redemption process and avoid income taxes.
NOTE: A federal employee is anyone who receives BENEFIT from the federal government. The maxim of law states "He who receives the benefit must derive the detriment." If one is on federal land and makes money from the land's usage, that is a benefit. The detriment being a tax on income, SSI, etcetera; however, with lawful money redemption the income tax no longer becomes an issue even in these scenarios. Social Security taxes and any other additional taxes will still apply.