I know of no way to do it with the annotation editor. However, you can achieve mirrored text with cgm export. Create the needed text and export that as a cgm file (there will be an option to export text as geometry). Import your text cgm file, all the text will be regular geometry now (lines, arcs, and/or splines) and will mirror easily. If you are going to use this text for extrusions or other operations it will probably need some cleanup first.
This tip works with NX2, later versions were supposed to add support for TrueType fonts. I'm not sure how or if the new text functionality changes things.