How to paint an area of a bitmap with 'transparent white'?

If you use the composite painting methods, then the alpha will be used to blend the colour, so nothing will happen.

If you want to set the bitmap, either create it from data with the background you want, or set the background using LockBits to manipulate the data en-masse.

You also might be able to do in using a bitblt method with the appropriate flags, but I don't know how to translate that to managed code.

You'll have to set the Graphics.CompositingMode property. For example:

protected override void OnPaint(PaintEventArgs e) {
    var img = Properties.Resources.Chrysanthemum;
    e.Graphics.DrawImage(img, 0, 0);
    e.Graphics.CompositingMode = System.Drawing.Drawing2D.CompositingMode.SourceCopy;
    using (var br = new SolidBrush(Color.FromArgb(0, 255, 255, 255))) {
        e.Graphics.FillRectangle(br, new Rectangle(50, 50, 100, 100));
    }
}

The actual color you use doesn't matter, you'll get a black rectangle with an alpha of 0.

I believe that you need to use SetPixel (or equivalent method of setting the color values directly) to get the pixels to be "transparent white".

You can use the Graphics.Clear method to set the color or pixels, but you can't use it to set them to both transparent and a color. I tried this to set the pixels in a part of a bitmap:

using (Graphics g = Graphics.FromImage(theBitmap)) {
  g.Clip = new Region(new Rectangle(10, 10, 80, 80));
  g.Clear(Color.FromArgb(0, Color.White));
}

The pixels in the region end up as "transparent black": 0,0,0,0. Even drawing a solid white rectangle before clearing doesn't help. When the alpha is zero in a color, the other color components are also zero.

Using an almost transparent alpha like 1 works fine, the pixels end up as "almost transparent white": 1,255,255,255.

Important tip:

Make sure when you create the image you do this

mainImage = new Bitmap(totalWidth, maxHeight, PixelFormat.Format32bppARgb);

and not

mainImage = new Bitmap(totalWidth, maxHeight, PixelFormat.Format32bppRgb);

It'll save you some trouble. Don't assume 32 bit means alpha channel ;-)