How to run a file preview handler through the surrogate host

Question

According to the docs:

Preview handlers always run out of process. There are two methods of implementing this:

1. A preview handler can be built as an in-process server but run through an out-of-process surrogate host. This is the preferred method. The system provides a surrogate host for this in the Prevhost.exe file. Preview handlers built by this method are not compatible with Outlook 2007 on Windows XP. However, these same handlers will work in Windows Explorer and Outlook 2007 running on Windows Vista.

2. A preview handler can be built as a local Component Object Model (COM) server. This is not recommended for several reasons. First, implementation of an in-process server is easier. More importantly, implementation as an in-process server provides greater control over the lifetime of the handler object, which allows for better cleanup and efficiency.

How does a host go about running a preview handler the first way? I've tried instantiating a preview handler like so, but unlike the preview handler in Explorer, this does not result in the launching of the Preview Handler Surrogate Host:

HRESULT PreviewHandlerFromExt(LPCWSTR pszExt, IPreviewHandler** ppph)
{
    WCHAR szCLSID[CLSID_LEN] = { 0 };
    DWORD cchOut = CLSID_LEN;
    HRESULT hr = AssocQueryString(  ASSOCF_INIT_DEFAULTTOSTAR,
                                    ASSOCSTR_SHELLEXTENSION,
                                    pszExt,
                                    L"{8895b1c6-b41f-4c1c-a562-0d564250836f}",
                                    szCLSID,
                                    &cchOut );
    if (FAILED(hr))
    {
        return hr;
    }

    CLSID clsid;
    hr = CLSIDFromString(szCLSID, &clsid);
    if (FAILED(hr))
    {
        return hr;
    }

    CComPtr<IUnknown> punk;
    hr = punk.CoCreateInstance(clsid, NULL, CLSCTX_LOCAL_SERVER);
    if (FAILED(hr))
    {
        return hr;
    }

    CComPtr<IPreviewHandler> pPrevHandler;
    hr = punk->QueryInterface(&pPrevHandler);
    if (FAILED(hr) || !pPrevHandler)
    {
        return hr;
    }

    return pPrevHandler.CopyTo(ppph);
}

Do you need a different flag for CoCreateInstance, or is there some other method you need altogether? Trying to instantiate handlers for Office files in proc always fails.