There would be a few levels of complexity to it. But if you're hosting a lemmy instance already, it shouldn't be any trouble for you ... basically make yourself the only account but allow people to federate with your instance. Add your own modified front end too if you like (as lemmy has separate backend and front end software stacks AFAIU). Interestingly, I think it would be a cool project for people to work on ... a front end suitable for hosting a single (or even multi) user blog on the fediverse.
An additional option would be microblog: https://docs.microblog.pub/. It's a single-user fediverse platform written in python and relying on sqlite (which sounds to me like a nice sweet spot for single-user instances).