Thanks for the reply. I can see why HAOS would be stripped down and locked down, since (it seems) it’s effectively an embedded system and for reliability one would want to limit or eliminate random effects on HA from other software running under the same OS. E.g. you wouldn’t want someone to be sporadically running a resource-hungry applicationon the same machine, etc.
For this reason I’m not keen to run it in a VM or container because then the environment HA is in (outside the VM/container) is variable in a similar way, dependant on the other loads the machine is subjected to. I understand it’s possible for this to work OK, but it does increase the number of permutations that apply. There do seem to be a fair number of questions about aberrant behaviour in the VM or container contexts (although others are clearly having no issues). I cba to get into all that, tbh.
So, for reliability, an exclusive install seems favourite. You just get the curated content and the outcome is as predictable as possible. Because HA ha exclusive use of the platform, there’s nothing else competing for resources.
However, I can’t see why a dual boot installed HA wouldn’t work, if it could be achieved by somehow flashing the HA image into the appropriate partition. Once the BIOS and bootloader have run, the respective OSs in the dual boot have exclusive use of the platform and all its resources as if the other OS was not there (OK, there’s a partition that’s not available to each OS but ignoring that). So, if HA could be put in a separate partition somehow, and the bootloader arranged to launch whichever file is the first thing to run in HA, then HA would not “know” there was a W10 install on the same disk. Unless HA had a requirement to be in the first partition of the disk, there should be no difference to it, regardless of whether it’s in a dual boot setup or exclusively installed. Becasue it’s based on Debian, this seems unlikely.
As koying said above, the HA “partition” for my weird requirement could be another physical disk, launched by GRUB or just selected in the BIOS, so I could do my pointless swich back to Windows at some time. I don’t think a separate partition on just one disk is any different - the problems are only how to get the partition established, how to make the flash program put the HA image in there, and how to edit GRUB so it calls the appropriate file on that partition to boot HA (assuming that’s how it works).
I actually think that possibly if I were to install Debian dual boot, then successfully flash over the debian install with HA (somehow), it would just work - because the file that is launched first would be the same in each case. I might give that a go.
Soooo, I realise totally I have a minority interest here . . . I’ll have a play around with it and see if I can make it work. Or give up and use two disks 
Thanks again
Rob