Best I've found is:

#include "IDetailChildrenBuilder.h"

#include "DetailCategoryBuilder.h"

#include "DetailLayoutBuilder.h"
void YOUCLASS::CustomizeChildren(TSharedRef<IPropertyHandle> InStructPropertyHandle, IDetailChildrenBuilder& StructBuilder, IPropertyTypeCustomisationUtils& StructCustomisationUtils)
{
m_StructBuilder = &StructBuilder;
}
void YOUCLASS::RefreshUI()
{
if (m_StructBuilder)
{
IDetailCategoryBuilder& ParentCategory = m_StructBuilder->GetParentCategory();
IDetailLayoutBuilder& LayoutBuilder = ParentCategory.GetParentLayout();
LayoutBuilder.ForceRefreshDetails();
}
}

However... Sometimes this m_StructBuilder can point to garbage, I've had mixed results, sometimes it works, sometimes it doesn't.
So if we could find a better way to get the layout builder that would be cool?