Это не поддерживается
[msdn]
[Note] Note:
Windows Forms controls are not designed to be marshaled across application domains. For this reason, Microsoft does not support passing a Windows Forms control across an AppDomain boundary, even though the Control base type of MarshalByRefObject would seem to indicate that this is possible. Windows Forms applications that have multiple application domains are supported as long as no Windows Forms controls are passed across application domain boundaries. [/msdn]

поддержка только в WPF и Framework 3.5 support-for-windows-forms-in-hosts-and-add-ins