If I had a license right now I could give you access to a dev. installation. There might be someone kind to do the same.. but I doubt you can get the licence for free. As a last resort you can try Contact Us form.
Given the current issues TinyMCE has, KAM would be not very smart to not give you a free license in exchange for a better editor. As suggested, I recommend emailing them with this contact form: http://xenforo.com/contact/
Might wanna edit that link out, other people can view and post to your ticket. I'm not very surprised though. I don't have many big complaints about TinyMCE, I would expect that most that do are XF "power-users." If creating an addon for the community entitled you to a free license I bet many developers here would be quite upset.
Not to discourage any potential developers. Might wanna get in touch with Nasr if he's offering you a license.