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.
I downloaded as much of the source of Bluebottle as I can. I'm glad the OP gave me the link to the modern one. I'll get it too. Think I might try to retarget it in one of the safer languages, secure hardware (eg Cambrige CHERI processor), or certified compilers. Oberon is the kind of thing a one-man project can make progress on. On other side, I think I recently saw another team quit after little progress on rewriting NetBSD in something safer. ;)