acme.sh offers a NameCheap plugin for the acme.sh ACME client. Maybe you could modify that script so it can be used as a --manual-auth-hook and --manual-cleanup-hook?
Other options are using acme-dns in combination with one of the following:
- Running your own RFC2136 capable DNS server in combination with the
certbot-dns-rfc2136plugin; - Use the
certbot-dns-standaloneplugin which has an embedded DNS server included; - The
acme-dns-certbot-joohoiscript which can be used as--manual-auth-hookin certbot.