I’ve wondered that myself, and I don’t know, to be honest, but there are some issues you’d certainly encounter. For example, if you posted any media it would need to be somewhere “always on” or remote instances and users might not be able to see it unless they managed to cache it on time. It means that your posts URLs wouldn’t be accessible, and would only be available on servers to which it has already federated. There may be other issues, too, such as queues only keeping undelivered messages for so long, etc.

I’m sure someone with a good understanding of ActivityPub could explain whether or not this is possible.