Best way to simulate "password char" system

[ItsTheSky]

Hi there!
I'm looking toward hiding part of a line, similarly to visually replacing a text with passwords char (• or *).
What is the best way to do that? Here's my "base code" so far:

public class TokenMasker : DocumentColorizingTransformer
{
    protected override void ColorizeLine(DocumentLine line)
    {
        if (!line.IsDeleted)
        {
            var lineText = CurrentContext.Document.GetText(line).TrimStart();
            if (!lineText.StartsWith("token: "))
                return;

            var start = line.Offset + lineText.IndexOf("token: ");
            ChangeLinePart(start, line.EndOffset, ApplyChanges);
        }
    }

    private void ApplyChanges(VisualLineElement element)
    {
        // what to apply here?
        element.TextRunProperties.SetTypeface(...); // this do not work, see below
    }
}

• I tried using a custom font, to use in TextRunProperties.SetTypeface (with only dots glyph), but Avalonia is dumb when we have to load a font dynamically (as I cannot do it another way), and it simply doesn't work
• Is there a way to visually replace the text of the line? (without changing its actual content)
• Can I create my decorations somehow, to override the drawing characters?

I appreciate any help you can provide :)