I wanted to call out where some of the inspiration to do this came from, and also point out another project here on Hackaday that's touched on the topic.
Inspiration - GeckOS by André Fachat
The main inspiration for me was about a year ago when I saw the YouTube video that I've linked at the end of this log entry. It's a presentation by Glenn Holmer from a retro computing convention, in which he demonstrates a multitasking OS running on a 6502-based Commodore 64.
The OS in question is called GeckOS, and it's by André Fachat. Any time you search for things like this on a 6502 you're likely to end up on André's web site! He was trailblazing this kind of thing back in the 90s, on both the software and the hardware side.
My first thought when I saw this presentation was that there are some really simple things you could do in the hardware to make this work a lot better. And of course that's exactly what André did - it's not shown in this presentation, as the presenter is only using a C64, but André was building his own monster of a homebrew computer, with 6502s and other processors all slotting together, and at least at some stages he was using quite a versatile MMU.
André's system did have some elements of memory protection, including quite advanced features such as virtual memory (using page faults to page from storage), and read-only and no-execute pages. I'm not sure whether he implemented a privilege system though - from his designs it looks like untrusted code could break out of its sandbox.
I also wanted to point out this other project here on Hackaday that has some similar needs. I'm not sure if it's still in development, it was a couple of years ago: https://hackaday.io/project/98837-8-bit-portable-internet-enabled-computer
They are thinking along similar lines, in any case, and of course, referring back to André's excellent work as well.
If you're aware of any other interesting attempts at this, please do let me know. I'll probably still forge my own path, as this is all about the journey for me, but I'd still love to check them out!