If you accept to link to some WPF's assemblies (WindowsBase and PresentationCore), you can write a custom TextBox and use it in a WinForms implementation. WPF has some nice classes like the GlyphTypeFace (allows to load a font file and build geometries from glyphs), and GlyphRun (which allows to draw a list of glyphs - a text). But we can't use GlyphRun here because we want to be able to modify the geometry of some glyphs. So we need to manually get geometries and transform them.

Here is an example:

The Winforms code:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        // ElementHost allows Winforms to host WPF visual components
        ElementHost host = new ElementHost();
        host.Dock = DockStyle.Fill;
        host.Child = new MyTextBox();
        Controls.Add(host);
    }

The custom TextBox code:

public class MyTextBox: UIElement
{
    protected override void OnRender(DrawingContext drawingContext)
    {
        const string sampleText = "Sample String";
        const int sampleEmSize = 30;

        GlyphTypeface typeFace = new GlyphTypeface(new Uri("file:///C:/WINDOWS/FONTS/segoeui.ttf"));

        GeometryGroup group = new GeometryGroup();
        group.FillRule = FillRule.Nonzero;

        double x = 0;
        double y = sampleEmSize;
        for (int i = 0; i < sampleText.Length; i++)
        {
            ushort glyphIndex = typeFace.CharacterToGlyphMap[sampleText[i]];
            Geometry glyphGeometry = typeFace.GetGlyphOutline(glyphIndex, sampleEmSize, sampleEmSize).Clone();
            TransformGroup glyphTransform = new TransformGroup();

            if (sampleText[i] == 'm') // this is a sample, we just change the 'm' characte
            {
                const double factor = 2;
                glyphTransform.Children.Add(new ScaleTransform(factor, 1));
                glyphTransform.Children.Add(new TranslateTransform(x, y));
                x += factor * typeFace.AdvanceWidths[glyphIndex] * sampleEmSize;
            }
            else
            {
                glyphTransform.Children.Add(new TranslateTransform(x, y));
                x += typeFace.AdvanceWidths[glyphIndex] * sampleEmSize;
            }

            glyphGeometry.Transform = glyphTransform;
            group.Children.Add(glyphGeometry);
        }

        drawingContext.DrawGeometry(Brushes.Black, null, group);
    }
}

And here is the result on the WinForms:

Of course, there is some work to do, if you want to support edit, but this may get you started.