Maybe a bot that once every three months (or whatever time period you prefer) does a commit to the changelog that says "I am [not] still maintaining this project" and it sends you an email a week before to hit a toggle to remove the "not". Then in the readme you can put a paragraph saying "I use Maintaina-bot (tm), and I commit to clicking the button every three months as long as I'm maintaining this project".

It's a little silly, but I could imagine that if this become a common enough thing, people would start to trust it.