I am not sure. What is the use case here instead of using a Let's Encrypt certificate?
It does look like custom certificates are supported (https://github.com/basecamp/kamal/pull/969) but I do not know if validation is done or not on these, so it would need to be tested.
It does look like custom certificates are supported (https://github.com/basecamp/kamal/pull/969) but I do not know if validation is done or not on these, so it would need to be tested.