Use and properties of characters above U+10FFFF

Chicken allows for characters with values beyond what the Unicode 
standard allows, like #\x1FFFFF. They have some weird properties (on 
6.0.0-rc3):

* (string #\x1FFFFF) ; -> "\0"
* (let ((p (open-output-string)))
     (write-char #\x1FFFFF p)
     (get-output-string p) ; -> ""
* (string->utf8 (string #\x1FFFFF)) ; -> #u8()

Is there any purpose or use for them?

-- Peter McGoron