gistfile1.js

// Top level component
<component>
  <UserModal isOpen={this.state.whatever} />
</component>

// UserModal

{
  render:function() {
    return <BaseModal {...this.props}>
      content
    </BaseModal>;
  }
}

// BaseModal

{
  mixins:[OverlayMixin],
  
  renderOverlay: function () {
    if (!this.props.isOpen) {
      return <span/>;
    }

    return <Modal {...this.props}>
      {this.props.children}
    </Modal>;
  }
}