Need to use string-to-codepoint in an RTF template

Hi. Some context: I'm a newbie trying to generate a check digit for an invoice barcode in an RTF Template.  Pity me.I need to convert some characters to their numeric values, similar to the Microsoft Excel CODE function, or the XSLT string-to-codepoint function.  Is there some way to do this, short of hard-coding it all into some monumental Choose-When ?