Will you support (allow) custom ACME clients/implementations?

Will you allow custom client implementations - such as for tight integration into existing tools or to handle scenarios such as requesting certs for internal servers - where I can control a DMZ server to put up the validation content in an automated fashion, but could not provide automated access to the system actually getting the cert?

Yes, we particularly encourage hosting providers to develop their own ACME clients.

You’re welcome to develop an ACME client of your own for the scenario that you describe, or, if you’d like, to contribute an authentication plugin to the Let’s Encrypt client that can help other people achieve what you want.