There is an extensions method for IContent called Ancestors, which I also believe gets automatically index. I suppose you could use that.
Something like this:

SearchClient.Instance.Search<SomePageType>().Filter(x => x.Ancestors().Match("1"))

Ah, you are using cms 6. Then that won't do you much good  :-\ 

It should not be too much code though:

        public static IEnumerable<int> GetParentIDs(this PageData page)
        {
           // get parent ids here

        }

and then

SearchClient.Instance.Conventions.ForInstancesOf<PageData>().IncludeField(x => x.GetParentIDs());

I resolved it using the following property in my basepage:

public IEnumerable<string> PagePathArray
{
get
{
return _dataFactory.GetParents(PageLink).ToString().Split('.');
}
}

Then the filter itself looks like this:

public ITypeSearch<IBasePageData> SearchRootFilter(ITypeSearch<IBasePageData> searchQuery, int searchRootId)
{
if (searchRootId > 0)
return searchQuery.Filter(x => x.PagePathArray.Match(searchRootId.ToString()));
return searchQuery;
//To use more than one searchRoot, use an .OrFilter();
}