Rewrote solution provided by @corylulu and @antgraf to add tracking of enabled changed events from the TabPages to redraw the TabControl:

public class DimmableTabControl : TabControl
    {
        public DimmableTabControl() {
            DrawMode = TabDrawMode.OwnerDrawFixed;
            DrawItem += DimmableTabControl_DrawItem;
            Selecting += DimmableTabControl_Selecting;
            ControlAdded += DimmableTabControl_ControlAdded;
            ControlRemoved += DimmableTabControl_ControlRemoved;
        }

        private void DimmableTabControl_DrawItem(object sender, DrawItemEventArgs e) {
            TabPage page = TabPages[e.Index];
            using (SolidBrush brush = new SolidBrush(page.Enabled ? page.ForeColor : SystemColors.GrayText)) {
                e.Graphics.DrawString(page.Text, page.Font, brush, e.Bounds.X + 3, e.Bounds.Y + 3);
            }
        }

        private void DimmableTabControl_Selecting(object sender, TabControlCancelEventArgs e) {
            if (!e.TabPage.Enabled) {
                e.Cancel = true;
            }
        }

        private void DimmableTabControl_ControlAdded(object sender, ControlEventArgs e) {
            e.Control.EnabledChanged += DimmableTab_EnabledChanged;
        }

        private void DimmableTabControl_ControlRemoved(object sender, ControlEventArgs e) {
            e.Control.EnabledChanged -= DimmableTab_EnabledChanged;
        }

        private void DimmableTab_EnabledChanged(object sender, EventArgs e) {
            Control control = (Control) sender;
            control.Parent?.Refresh();
        }
    }