WebControl development: how do I allow content in a webcontrol that subclasses a control that doesn't?

Question

I'm subclassing (among others) System.Web.UI.WebControls.RadioButton. I want to allow child tags for configuration, like so:

<my:RadioPlus runat="server">
    <option attr1="something" attr2="somethingelse"/>
    <option attr1="something2" attr2="somethingelse2"/>
</my:RadioPlus>

The RadioPlus class is decorated with [ParseChildren(true), PersistChildren(false)], and the has an Option property decorated with [PersistenceMode(PersistenceMode.InnerProperty)].

This works at runtime, but Visual Studio 2008 is complaining at design time that "Content is not allowed between the opening and closing tags for element 'RadioPlus'."

How do I inform Studio of what I'm doing here?