They've publicly acknowledged that they're still working but have been quiet.
> I imagine it’s very frustrating to not have much to show after more than half a decade of development.
Yes and No. I think, rightly so, they are concerned about pre-mature commercialization. Tech transfer in HCI-like domains can often be wrought with issues:
"The Dynamic Medium Group’s vision is rooted in the idea that the computer revolution of the ’70s and early ’80s was cut short, primarily by premature commercialization. While the computer as a medium was still unfolding its potential, and way before it could do so entirely, it was solidified into commercial products, thereby stifling its free growth. Once corporations had built their businesses on the ideas developed so far, they were only interested in incremental change that could easily be integrated into the products, rather than revolutionary new ideas.
Because of this cautionary tale and to avoid repeating history, the group is wary when it comes to public attention and deliberately reserved in what it shares. "
However, I do think that getting ongoing feedback & recognition of impact is important even in long-term research efforts. This was hinted at in Glen's note on why he left Dynamicland: http://glench.com/WhyIQuitTechAndBecameATherapist/
The saddest observation through all this is ... why don't academia or corporate research labs do this type of long-term, personal computing focused research anymore? It's never been easier to build a computer or an operating system, but nobody has the appetite for it anymore. Maybe people feel that operating systems and computers in their current form are here to stay forever. Maybe the most we can do is port things clumsily to VR or AR.