I've just spoken to Sonja about this. Please see below for the end of an email she sent me this morning regarding the copyright status of the language.
Angela.
---------- Forwarded message ---------- From: Sonja Elen Kisa marraskuu1978@yahoo.com Date: Wed, 22 Sep 2004 00:45:44 -0400 (EDT) Subject: toki pona Wikipedia To: angela@wikimedia.org
[...]
Anyway, the Toki Pona language is obviously released under the GFDL.
Sincerely,
Sonja