I know how to configure VS 2008 to open windows forms controls in code instead of the designer. But I think I have seen a way I could do this with an attribute I could decor
This option is available in Visual studio 2010
I think I have found where you already seen that: the Brian Ensink's blog. You can use the DesignerCategory attribute to mark class in order to choose how they are opened:
The magic line you seek is:
[System.ComponentModel.DesignerCategory("Code")]
Just put that before your control-derived type and kiss that useless designer goodbye.