A private beta tries to allow the programmer what the name suggests - privacy. ThunderZ has his reasons for wanting privacy. It doesn't even mean he won't release his source code at some point, it just means he wants to code in peace for the moment.
I can't really fathom exactly what ThunderZ has experienced, as I have never written an emulator for something like the GP32. Yet, I can guess what he is going through. ThunderZ wishes to code like a rabid dog trying to get his/her emulator complete enough to his own satisfaction. They are his own personal projects, so its no wonder he wants them up to his own standards first. People stalking him on message boards and email bombing him with questions, "new features to add", etc. are only a distraction, because at this early in the development stage, it's mostly about functionality and not about compatibility. Therefore, its not even "beta testing", its more like "alpha testing" at best.
In short - Don't ask for a public beta. ThunderZ obviously has his/her reasons. He's/she's the great emu author. Let him code in peace.