Illegible handwriting may be commonplace, but one normally thinks of asemic writing, and of pseudobiblia, as exotic and maturely experimental and vaguely avant garde. Then again...

Adult handwriting was unreadable, true, but perhaps that was its point. I saw the loops and impatient dashes of the adult hand as a secret and untrustworthy way of communicating that one day I would master. Unable to bear it any longer, I took a pen and covered a whole page of my school exercise book with grown-up writing, joined-up writing. There were no letters there to be read, still less words; just diagonal strokes linked each to the next in a bold series of gestures. That, I thought, was grown-up writing, if only it could be made to mean something, too.
Philip Hensher, The Missing Ink, p. 6

Longed to start on joined-up handwriting. Invented own script, bearing no relation to letters or words. Developed inexplicable urge to write under the pseudonym of Edwin Harrington... For some time wrote title pages of unknown novels by Edwin Harrington, my chosen pen-name, with absurd arabesques under the signature. Now eight years old. The novels of Edwin Harrington went unwritten.
Ibid, pp. 33–4

It would seem that before he was nine Hensher had independently reinvented the conceits of Borges, Xu Bing, maybe Luigi Serafini, and, possibly, whoever wrote the Voynich Manuscript. Either he was some manner of prodigy, or such creations may be more commonplace than might have been supposed...