Transcode strings into Numeric Character References?

Just wondering what the quickest way is to transcode strings into
numeric character references? I need to ensure that all "strange"
characters (curly quote marks, etc) are encoded, and for some items I
need to encode alphanumeric characters also. However, I'm using XPath
1.0, and don't fancy doing a heafty xsl choose statement to loop through
each element.