Thank you for your reactions. I couldn't keep up today, had to put in some hours of actual work :-)
When I first asked about "hiding" OS partitions from each other in a (UEFI) multiboot scenario like the one I tested, Wonko suggested simply removing the drive letters. This was actually a very good idea, it's always good to keep things simple if possible. So I tested that method, and after learning how to set the "0x8..." attribute, I finally figured out how to prevent letters being "messed up", now I know how to check what was going on, I believe I mentioned that earlier.
I will probably try out actually *hiding* GPT partitions as well, but my first (quick) tests didn't yet show a real advantage to this "hide" attribute, it seems to me the partitions are still visible in Disk Management and Diskpart, so to me that doesn't seem be a whole lot more useful than simple "having no drive letter". I',m sure some of you will know more about this.
Alacran, thanks for the Bootice recommendation, in fact I use Bootice a lot already, both in command-line mode and GUI mode (it even has its own hotkey in my Autohotkey shell). On MBR multiboot OS installs, I use the silent command-line mode to re-install Grub4Dos after a new OS setup (in my Postinstall system). It never failed me.
I think the reason for my first idea being to somehow hide the (GPT) partitions was because this was so easy to do with the "old" Grub4DOS on MBR. But once again: simply removing the letters (and knowing why they might reappear or not) is a workable solution.
I did manage to test some simple .ahk code yesterday, that will later be included in my Apply scripts (I never use standard Windows setup from ISO, I think I used that once, after that only WinNTSetup and later on my own PE scripts).
As long as the "OS" partitions (p2, p3, p4, p5) retain their "0x8..." attribute, in PE there should only be two drive letters issued: C for the first Data partition, D: for the second one (*). If this is not the case, I can always display a message like "The drive letter assignments are not as expected, please check this manually with Disk Management".
(*) In that scenarion, it's pretty easy to [from a script, of course] rename D to E, then C to D, after that letter C is free to give to one of the OS partitions, of course reset "0x8..." attribute (**), then apply OS. Works great!!!
(**) That was the whole point :-)