So, this code worked for me to get the contents of the clipboard in the Art::Text clipformat, modify it, and paste it back in with a different text and "field type". The custom field type shows up in the PPTX file after saving.

I have no idea if this is "safe" or compatible with anything at all - you should probably read the documentation of the drawingml format to figure that out.

 var input = System.Windows.Clipboard.GetData("Art::Text ClipFormat") as MemoryStream;

// Take a copy of the data into a writable stream
var strm = new MemoryStream();
input.CopyTo(strm);
strm.Position = 0;

using (var p = new ZipArchive(strm, ZipArchiveMode.Update, leaveOpen: true))
{
    var drawingEntryName = "clipboard/drawings/drawing1.xml";       
    var drawing = p.GetEntry(drawingEntryName);
    XDocument doc;
    
    using (var drawingStream = drawing.Open())
    {
        doc = XDocument.Load(drawingStream);
    }
        
    var NS_DrawingMl = XNamespace.Get(@"http://schemas.openxmlformats.org/drawingml/2006/main");
    var fieldXmlNode = doc.Descendants(NS_DrawingMl + "fld").Single();
            
    fieldXmlNode.Attribute("type").SetValue("mycustomtype2");
    // Replace the ID to avoid two fields with same ID in document. Might cause trouble? I don't know.
    fieldXmlNode.Attribute("id").SetValue(Guid.NewGuid().ToString("B").ToUpper());
    
    var tNode = fieldXmlNode.Descendants(NS_DrawingMl + "t").Single();
    tNode.Value = "21.05.2021"; 

    using (var drawingSaveStream = drawing.Open())
    {
        doc.Save(drawingSaveStream);
    }
}

strm.Position = 0;
var clipboardData = new DataObject("Art::Text ClipFormat", strm);

System.Windows.Clipboard.Clear();
System.Windows.Clipboard.SetDataObject(clipboardData);