I installed the (razor) package in v6 but got no pages in the sitemap (just the empty <urlset> tag.

There seems to be some problem (bug?) with the Linq parsing of the Where clause for umbracoNaviHide.

To get it working, I done this:

Before:

 foreach (var node in startNode.Children
        .Where("HasAccess")
        .Where("!IsProtected")
        .Where("umbracoNaviHide == false")
      )
    {     
        if(node.template > 0) {
            <url>

After:

foreach (var node in startNode.Children
        .Where("HasAccess")
        .Where("!IsProtected")
      )
    {     
        if(node.template > 0 && !node.umbracoNaviHide) {
            <url>

I also had to add the () to the Count method as described by Dan in this thread. 

if (node.Level <= maxLevelForSiteMap && node.ChildrenAsList.Count() > 0)

Hope this helps!

-tk