Actually, the FormattedTextEditor does support this.

Here's some quick sample code that demonstrates how you might do it:

            this.ultraTree1.Override.UseEditor = Infragistics.Win.DefaultableBoolean.True;
            this.ultraTree1.Override.EditorControl = this.ultraFormattedTextEditor1;

            this.ultraTree1.Nodes.Add("ABCDEFG");
            this.ultraTree1.Nodes.Add("AB<span style=\"background-color:Yellow;\">CD</span>EFG");

We have a formatted text control (UltraFormattedTextEditor) but I don;t believe it supports applying a highlight to characters (you could however use that control if you wanted to change the foreground color). The only way to do this that I can think of is to use the IUIElementDrawFilter interface.