@lawrie Thanks for the tip! In the mean time it runs a much enhanced version of V6 with elements taken from V7, V8 and V9 and the TCP/IP stack from BBN (that ended up in 4.2BSD). This is what I want to get running on the ULX3S. It will also be the machine that talks to the Blit, running on another ULX3S.
For a while in 2017/2018 I was planning to do a similar SBC with a 99105 or 99110 CPU and a better MMU. But then @Speccery convinced me that FPGA's were perhaps a more interesting route to go. It took me a long time to overcome my reluctance to a multi-gigabyte vendor tool download, but took the plunge. Luckily Trellis was just around the corner, as was the ULX3S.