Caddy/CertMagic will probably have a CLI command (or admin API endpoint) you can invoke to get the value(s) needed for your dns-persist TXT records. It won't publish the record for you (for reasons already stated above; the point is not needing to provide DNS credentials).
We'll probably have users configure the ACME client to use the dns-persist challenge but let the ACME server do all the verifications.
I might experiment with trying to auto-detect dns-persist but I too worry that it will fall down at scale (be slow, not cover edge cases, etc).
getssl only supports the http-01 and dns-01 challenges. These are the most common so that is not surprising.
But, adding support for a third challenge dns-persist-01 will involve changing quite a few lines of code. If you want that supported long-term in getssl you'll need to provide that in a form the authors will be willing to include in the main build.
I apologize if I just misunderstand and you are an experienced coder. In that case, the reason Let's Encrypt is still validating using the dns-01 challenge is because you have not posted the URL for the dns-persist-01 challenge. The list of challenges appear in the authz object.