I could be wrong, as there are an aweful lot of character sets in the Unicode standard, but I don’t think they have an assigned codepoint. You would have to use the Private Use Area.
Do you know of any fonts with these symbols? I thought I had seen them somewhere, but I don’t see them now in Minion Pro. OpenType fonts will use Glyph Substitutions so that when a user types 1st 2nd 3rd 4th etc., the letters will be replaced with the superscripted glyphs. You will need to use other software to do this.
See this Tutorial thread: Adding OpenType Features
I found that Palatino Linotype has a full set of superscript lowercase letters, and none of them are mapped. They are used by OpenType Glyph Substitutions. These would only be available in InDesign or other programs that offer OpenType support.
Palatino Ordinals.png