I've answered my own question.
In the TreeView tag, use the DataBound event to loop through the nodes, find the node you want using node.Text, and apply a SPAN tag with style="" to control formatting.
ie:
protected void Tree_DataBound(object sender, EventArgs e)
{
try
{
foreach (TreeNode node in TreeView1.Nodes[0].ChildNodes)
{
if (node.Text.Equals("miJob Register"))
{
node.Text = "<span style='background-color:yellow;'>miJob Register</span>";
node.ToolTip = "This replaces the old Helpdesk system. Please use miJob Register to raise helpdesk requests as of 2015 onwards.";
node.
}
}
}
catch { }
}
Works a treat!