Well, it turns out that this was easier than I was thinking it was.

If you add a handler to the KeyDown event, you can catch it.
//========================================================
private void  richTextBox1_KeyDown(object sender, KeyEventArgs e)
{
           // We call Cut() and Paset() directly on Ctrl+X and Ctrl+V
           // then in WndProc, we will handle the cut and paste events.
           if(e.Control && e.KeyCode == Keys.V  )
           {
                 Paste();
                 e.Handled = true;
           }
           else if(e.Control && e.KeyCode == Keys.X  )
           {
                 Cut();
                 e.Handled = true;
           }
}

//========================================================
Before I tried this I was sure cut and paste behavior would remain
unchanged, but it actually works.