So the "trade secret" thing explains why you don't see a port of Plan 9 for every new and exciting device and board that comes out. It is a shame, but these companies producing things like the GPUs in ARM devices insist on proprietary licences.
As far as I can tell, the i.MX6 chip from Freescale is about as open as you can get, I'm just trying to remember where someone said it had an over 1,000 page manual for the chip, and I'm hoping its not this very list, otherwise I will look like a fool! The i.MX6, for that "openness" reason has been a chip I've been meaning to get, and never gotten around to. The UDOO board has the chip, and integrates an Arduino Due along for the ride. Its reasonably cheap too, although availability is not something I've looked at in a little while, and being one of those Kickstarter things, I don't think availability is a priority past shipping them to backers.
I found the blog I was referring to:
And it is a detailed read, I've skimmed over his Season 2 with interest. He cites your work regularly.
I think you can tell what I'm thinking by mentioning the i.MX6, but I'm not pressuring you to make a port, or even get a board with the chip on it. I just think it would be interesting having Plan 9 on a multicore ARM chip, but as I said, I'm pretty certain its open enough, but please do not quote me - I'd hate to be horribly wrong!
Many thanks for your description!