To clarify: this is to counter the statement that security through obscurity is a good thing. Yeah, that worked out well for the Enigma and countless other systems.
I imagine Prof. Wirth must have thought that 32032 wasn't going anywhere soon 20 years ago.
As I see it, the ARM instruction sets (note the use of plural) are a moving target (look at Cortex chips with Thumb2 only). They are far from RISC if you count them, and the instruction encoding is horrendulous.
It's like saying "why not target Windows? I doubt that's going anywhere soon"
With FPGAs, your only risk is that the manufacturer discontinues the chip you targeted, and you can then target another brand with the same (or nearly the same) HDL code.
It is foolish to assume that HSAs don't already have access to whatever you, some guy on the Internet, know. So you can be an asshat, or do the right thing and try to 'protect the muggles' by letting information be available to all. They might feel threatened enough to pull valuables from unsafe places (such as networked computers).
Oberon is a pretty interesting system to muck with.
Another approach is to blackbox USB and HDMI, and consider them outside the project, much like the VGA monitor or whatever is in the keyboard to send PS/2 signals. You can connect to spare IOs on the FPGA board with 'blackbox' hardware, as long as protocols are simple and open.
You really should publish, as security through obscurity only hurts us! Out in the open, there is a chance that some measures will be taken. Concealment is a guarantee that bad guys will be doing bad things quietly.
One of the goals of Project Oberon is a system that can be understood in its entirety by a single individual. A USB controller, or an HDMI encoder would take several books to describe.
There are hundreds of FPGA boards that do not meet the requirements of Project Oberon, in addition to the board you mention.