haven't gone through the trouble of registering copyrights with the government or with copyright policing services,"

This is country specific, of course. For example, in Australia the act of committing a work to media of any form is enough to register and assert copyright for that work.

So as an Australian resident I am quite within my rights to assert the following immediately, with the legal backing of the Commonwealth of Australia:

This post is copyright ©️ dgriffith. I grant the Fediverse and its users/instances permission to duplicate as necessary as part of the normal mechanisms of participation, all other rights are reserved.