Select hidden treeviewitem in databound treeview

I'm having an issue selecting a databound treeview item in a treeview.

If the treeview has not already been expanded, then the treeviewitems don't
exist, so the code:

TreeViewItem tvi =
(TreeViewItem)(header.ItemContainerGenerator.ContainerFromItem(databoundobject));

returns null.

If I do:

header.IsExpanded = true;

before the call, the UI isn't updated (and the items bound) until after my
method returns, so I still get null (but the treeview is expended after my
method returns).

So, I tried using a WorkItem to expand on another thread:

if(header.IsExpanded == false)
{
header.IsExpanded = true;
//we have to queue a workitem since we have to wait for
the header to expand

System.Threading.ThreadPool.QueueUserWorkItem(ExpandedToSelectProcessWorkItemCallback, databoundobject);
}

However, then my callback throws a InvalidOperation exception because I'm
accessing the treeview on another thread.

So, I dug around and found people talking about the UIContext in this NG,
but I can't find any mention of it in the Dec CTP.

So, my question is:

-What's the best way to expand the treeview before searching it? (Please
don't say animations, please don't say animations.. )
-Bonus Question: what's the equivalent of UIContext in the Dec CTP?