Can treasure hunters NOT working for an organization Be Lawful in alignment?

[The Glyphstone]

I might agree with reliability, but "honor" and "trustworthiness" seem like traits common across Neutral Good and Chaotic Good.

Honor is a Good trait to have. Trustworthiness is more of a Lawful thing - and Neutral Good can exhibit lawful behavior...even Chaotic Good can be Lawful on occasion, they just don't behave so in all aspects of their life.