If using a derived node type, you could implement that by overriding Draw and DrawHandles:

public override void Draw(IGraphics graphics, RenderOptions options)
{
	if (this == Parent.ActiveItem)
		base.DrawHandles(graphics, Parent.ActiveItemHandlesStyle, options);
	else if (Selected)
		base.DrawHandles(graphics, Parent.SelectedItemHandlesStyle, options);

	base.Draw(graphics, options);
}

public override void DrawHandles(IGraphics graphics, HandlesVisualStyle style, RenderOptions options)
{
} 

For standard nodes you could draw handles in the background by handling DrawBackground event and calling Selection.Draw:

private void diagram_DrawBackground(object sender, DiagramEventArgs e)
{
	diagram.Selection.Style = SelectionStyle.SelectionHandles;
	diagram.Selection.Draw(e.Graphics, new RenderOptions());
} 

but then you will have to stop standard handle rendering in some way, e.g. by assigning transparent brushes to ActiveItemHandlesStyle and SelectedItemHandlesStyle.

I hope that helps,
Stoyan

This should show the mouse-over handles too:

public override void Draw(IGraphics graphics, RenderOptions options)
{
	var view = options.TargetView as DiagramView;
	if (view != null && view.AutoHandlesItem == this ||
		this == Parent.ActiveItem)
		base.DrawHandles(graphics, Parent.ActiveItemHandlesStyle, options);
	else if (Selected)
		base.DrawHandles(graphics, Parent.SelectedItemHandlesStyle, options);

	base.Draw(graphics, options);
} 

I hope that helps,
Stoyan