How to assign glyphs to control characters?

Question

It seems XML doesn’t accept values in range from�toinside attributes but allow#x7fto¡. It trigger that kind of error in my parser :

parser error : xmlParseCharRef: invalid xmlChar value 18
<glyph glyph-name="uni00000F" unicode="&#x12;"
                                       ^

I also tried opening the file in several web browsers and got similar error messages for the same lines.
I couldn’t find in the standard the purpose of this behaviour. It seems however that escaped Unicode control characters are allowed.

So does someone know the reason of that design ?
Is there an alternative for defining and using those glyphs outside an svg file ?