This an interesting thread, though I'm a little lost, as I can read again, but skim reading. I had also written something up to post about a modern representation of what could have been done with the QL myself, but the login authorisation too so long other things came up and I got sick. I want to look at it to see if it's worth posting. I've determined not to do it myself.
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On this thread. I may have a few suggestions. I found it googling Smallest Sinclair QL. It occurs to me that the QZero could be a basis for such a machine, with the modifications I mention in my thread. The Commodore 64 HP serial bus was ahead of it's time, a space that USB and Thunderbolt now occupy. But, such a scheme was viable on more condensed logic than the previous generation Commodore attempt, and healthy profits for the company selling the peripherals. So, a QZero plus USB, HDMI over USB SD card and basic sata (or whatever replaced it) and few other interfaces, represents a complete system in the space a pi zero W would take up. In today's terms there are a lot of open hardware out there, and graphics gpu softcores, with their own drivers. But, all that is needed for a cheap machine is the USB soft cores drivers and a few more graphics modes (and that blitter).
Another idea, that might be of help developing drivers and apps, that I had years back, was to vsee things on another platform which offers services. Android uses linux and the hardware drivers which come with that. I understand the universal driver attempts on Linux failed, but android's core exists. Linus got his start from owning a QL, and a android Linux core, would be a natural relationship.
For software, a JavaScript engine, like Firefox OS and Chrome OS did, but local installed desktop applications instead, to maintain web compatibility and offer a way to write and get cross platform applications. When JavaScript is not good enough, native code is used (there is also a way to run Linux apps through a framework on JavaScript).
Using these approaches, after the original system is implemented existing drivers and apps, hardware soft cores, USB libraries and whatever services attached to the Linux kernel under android, can be used from elsewhere.
I had also been looking for a Sinclair QL on Pi Pico emulator. I have found various 8 bit Sinclair, Jupiter, Commodore, Atari systems, and the Nes. But, the QL is not far off of an 8 bit system, and does the Pico use internal 32 bit data which would make emulation easier. A high speed method of emulating systems, is to translate the code to local machine language. Except for lack of graphics output, the Pico offers a reasonable minimal target system. I do not like the whole stick a Pico on a base board to make an emulated system, when they could be using the Pico and Pi chips themselves as the basis of the base boards.
Manufacturing and legal protection, certification, IP, warranty, distribution are very complicated before you even do the business. There must be a better way to get this all done, to take it off of our hands.