As I understand it, the relations department HAS to do the inviting. I don't recall if there were and laws pertaining to this, but I wouldn't be surprised if there was something. What you can do is this: if you have the contact information for anyone that you think might make for a guest at the con, give the relations department their contact info; they will handle the details, which includes a contract (hence I am sure that there are laws around this very subject).
I personally don't understand the thrill of "celebritizing" cosplay groups, but that's in addition to the fun of conventions. The wider variety one has, the wider the audience it can draw. Honestly, if a con JUST had voice actors, it'd probably get boring, even for someone like me who prefers to meet them over fellow fans who are "famous" for being fans.
If you'd like, the email is relations@kumoricon.org . Good luck!