Can you sign and notarize your own software made for internal use with your own infrastructure? If so, then this is a valid response. If not, then this is an irrelevant response because the issue is going through Apple, not the process being difficult or undocumented. If I own the device, then I should be free to decide what the sources of authority over it are.

Edit: I haven't tested it yet, but it does seem that you can sign an executable with your own certificate (self-signed or internal CA-issued) however you can't notarize it. Right now, notarization is only required for certain kinds of Apple-issued developer certificates, but that may change in the future.