Don't hold your breath, it's a Japanese company; updates and new products are few and far between.
I think this is a little inaccurate ... Japanese companies in general actually tend to update and change their products at a ridiculous rate (I'd argue
too often), because that's what works best in the Japanese domestic market, which tends to favor novelty and newness.
In the case of the HHKB, though, I think it's essentially a niche product that has existed on autopilot for a long time, and doesn't seem get much attention from the manufacturer (though it must sell OK if it's still on the market after 20+ years!).
[Another factor is that most Japanese products, including most of the model churn, don't tend to make it out of Japan; in the case of the HHKB, though, I never saw much activity in the HHKB line in Japan either.]