The first question that comes to mind is why you don't reverse the order of the characters and use a positive kern, but I played with with inserting the non-joiner special character between tow glyphs and found that I can kern a -1000 on each side of it, for an "apparent" total of -2 em.
Try the IndyFont demo :-) (http://forums.adobe.com/message/5268780).
This demo (there is a Full Version on its way!) only allows you to create a single character per font, but you can create as much fonts as you want.
(Out of curiosity: what sorts of special characters are you using?)