I am of a much lower level of experience than you, so I hope I'm not being pompous here, but the way I see it is that nothing is sacred, especially in martial arts. From what I've been taught, a martial artist mustn't be vain about their ability or hold themselves in a higher regard than other students. This "secret kata" concept seems to be teetering on vanity/narcissism, as though its creator feels their techniques are "too powerful" or "too exclusive" to teach to the masses, and that only they and a select few people are "worthy" to learn it.
Though I have to admit teaching with a reward system does have its perks; I remember back a few years ago working extra hard on my green belt test and feeling so excited when I finally got to learn Pyung Ahn Sam Dan (and then working extra hard on it because of how much I loved it).
In a nutshell, I see where you are coming from in regard to the reward incentive for kata, but keeping a kata "secret" seems fishy, not to mention somewhat impossible to do. This is especially true nowadays where I, for example, can easily go on YouTube and teach myself the Hwa-Rang Hyung (a "secret" Tang Soo Do hyung only taught to the highest-level masters, distinct from the ITF form of the same name).