with the version 8.20 we have enhanced the set of string() instant constructors to do what you expect. This however is not available in the old version 6.50. You have two possibilities:
Option 1: Update to the newest version (actually 9.10).
Option 2: Implement your own format function in Chora to convert a number in HEX string. The following example demonstrates it:
var uint32 number = ...
var string digits = "0123456789ABCDEF";
var string result = "";
while (( number != 0 ) || ( result.length == 0 ))
result = digits[ int32( number & 0xF )] + result;
number = number >> 4;