Can I use Direct Publisher to connect to an internal server

That's kind of what I was expecting to see.
With DP, I don't believe that will work.
Check out the DP specifications at this webpage
The JSON snippet from my channel that does work with DP looks like:

{
  "providerName": "This can be pretty much anything.",
  "language": "en-US",
  "lastUpdated": "2018-07-27T23:46:13+00:00",
  "shortFormVideos": [
    {
      "id": "1001",
      "title": "Name of videos",
      "shortDescription": "This would be a short description of the video that you want folks to know. ",
      "thumbnail": "http://domainname.com/path_to_thumbnail_image.jpg",     
'If you have an internal address this can also be an IP address.'
      "genres": [
        "comedy"  
'this has to be from the list of genres that roku supports'
      ],
      "tags": [
        "the60s"  
'this is how to identify specific playlists or ways that you want videos to play back'
      ],
      "releaseDate": "1960-07-24",
      "content": {
        "dateAdded": "2018-07-21T14:14:54.431Z",
        "captions": [],
        "duration": 105,  
'this is in seconds'
        "adBreaks": [
          "00:00:00"
        ],
        "videos": [
          {
            "url": "http://ip_address_or_domain_name.com/path_to_video.mp4",
            "quality": "HD",
            "videoType": "MP4"
          }
        ]
      }
    },
  ],
  "categories": [
    {
      "name": "Featured",
      "query": "featured",
      "order": "most_popular"
    },
    {
      "name": "Play them all",
      "query": "pre50 OR the50s OR the60s OR the70s OR pre2000 OR post2000",
      "order": "most_popular"
    },
    {
      "name": "1960 to 1969",
      "query": "the60s",
      "order": "chronological"
    },
  ]
}

That format, at least for a JSON, is pretty much non-negotiable. The order can of course change but all of that has to be there.

XML will of course look pretty different but it is still going to need everything listed in this URL for the XML (MRSS) Feed specs
Rovidx Media.
May the Roku DP be with you.