So, this is interesting. It turns out that there are practically enough public domain components on the Internet in 2018 to build a quite convincing minimal operating system.
20180529/http://invisible-island.net/byacc/byacc.html
20180529/http://re2c.org/
20180529/http://openxos.org/
broken/http://home.swipnet.se/smaffy/snafu/
20180529/https://pdclib.e43.eu/
20180529/http://pdos.sourceforge.net/
20180529/http://www.t3x.org/subc/
broken/http://www.cod5.org/archive/
20180529/http://charon.persephoneslair.org/~andrea/software/mpkg/