I mostly concur with jan Kipo
Word for word copies of a language inventor's book: bad, unless it was made available under say a creative commons license or GPL. The jan Pije lessons are public domain. jan Sonja material is mostly creative commons. (Elizar's book is CC, I think) So the most widely translated work is jan Pije's lesson and it is 100% safe to translate, transform or do what ever with that.
Translations of such a book into another language: bad. (Clearly a derivative work) [Again, unless it was release under public domain, creative commonts, etc)
Dictionaries. Dictionaries are the hardest to copyright because two competing dictionaries have largely the same content and the courts have said no one can claim to be the only person allowed to create a dictionary or say a phone book. I would assume a word for word, letter for letter copy would still be bad.
Original works in toki pona. You have copyright to them subject to where you posted them on the net. For example, the yahoo mailing list material-- yahoo has a right to use it, but otherwise is silent on what rights they retained. Material published on the fourm, we mostly think means you've released it under creative commons according to the notice on jan Sonja's front page. If you publish on your own website, of course, you still retain full copyright until you release it under CC, GPL or what have you.
The word toki pona. It isn't trademarked and the idea was unpopular. The lojban vs loglan fight was fought around trademark. The court decided at that point in time, loglan was a generic word, so loglan lost. Klingon is a trademark owned by paramount, the Klingon canon mostly falls under the same laws that cover the movie scripts. Paramount granted rights to the language to the KLI.
Translations of copyright works into toki pona: bad, but it would be absurd for copyright holders to pursue it. It still could happen. The relevant law would be the legal ideas being applied to fan fiction. If transaction costs were free, then everyone should ask for permission before translating and publishing copyrighted works. In practice, people merely try to credit the copyright holder and not make money off the results.
Conscripts, fonts and other artistic works This is a interesting case that I don't know the answer to. jan Josan's the best example of this scenario. I assume he has full copyright to the exact images and artistic words created with those images, and maybe no rights to imitators who are using a similar font of their own making to write new texts. It's on my to-do list to learn how copyright applies to fonts.