SiteMapNode Attributes - values being lost (SharePoint 2010)

Question

OK - I've created a custom navigation provider that inherits PortalSiteMapProvider and I'm trying to extend my class via the GetChildNodes method (Simple example here).

Essentially what I'm trying to do is add the current SPWeb's SiteLogoUrl string as an attribute (called imgurl) of each SPWeb's respective SiteMapNode, with a view to retrieve this value later in my custom Menu control (which inherits SharePoint:AspMenu).

I'm setting the attributes like so in my provider under GetChildNodes:

public override SiteMapNodeCollection GetChildNodes(SiteMapNode node)
    {
        PortalSiteMapNode portalNode = (PortalSiteMapNode)node;
        if (portalNode != null)
        {
            if (portalNode.Type == NodeTypes.Area)
            {
                SiteMapNodeCollection nodeColl = base.GetChildNodes(portalNode);

                using (SPSite currentSite = new SPSite(portalNode.PortalProvider.CurrentSite.Url))
                {
                    foreach (SiteMapNode topLevelNode in nodeColl)
                    {
                        foreach(SiteMapNode currentNode in topLevelNode.ChildNodes)
                        {
                            string currentWebUrl = currentNode.Url.Substring(0, currentNode.Url.ToLower().IndexOf("/pages/"));
                            using (SPWeb currentWeb = currentSite.OpenWeb(currentWebUrl))
                            {
                                if (!string.IsNullOrEmpty(currentWeb.SiteLogoUrl))
                                {
                                    currentNode["imgurl"] = currentWeb.SiteLogoUrl;
                                }
                            }                            
                        }
                    }
                }

                return nodeColl;
            }

Now I can debug this and retrieve the value once it has been inserted like so (x being the index of node, and me doing this in the Command Window):

? topLevelNode.ChildNodes[x]["imgurl"]

and that returns the url of the image fine : "/_layouts/images/myimage.jpg"

Now.. when I try to do this in my custom Menu control I first get a handle on my custom nav provider like so:

                CustomNavProvider customProvider = (CustomNavProvider)SiteMap.Providers["CustomNavProvider"];
                SiteMapNode currentNode = customProvider.FindSiteMapNode(childitem.NavigateUrl);

and the currentNode variable now contains my SiteMapNode...great, however instead of the attribute I added previously, there is now only a single attribute called 'AlternateUrl' - the imgurl attribute I added in the provider has now disappeared. All other properties like title, desc etc are there so it's definitely returning the correct node.

What's my issue here? Is the attribute not being persisted when I added it to the node in the provider?

Any help or suggestions appreciated.