Following suggestions from #plasma about qtVirtualKeyboard working with screen lock, I've been daring enough to try to put back locking mechanisms on my keyboardless tablet (Thinkpad Helix 2).
It is worth noting that I do not use Wayland partly because the tablet has a Wacom stylus digitizer. I'm happy to say the virtual keyboard was available, both at login time with sddm and to unlock the screen, as advertised. (The virtual keyboard doesn't seem to be available beyond that point; I use CellWriter.) This makes the tablet much more usable; and a very convenient tool indeed. I insert here a relevant, though off topic piece of information: because of a bug in the bios, you won't however be able to use usual suspend to ram, and will have to resort to suspend to disc, always. [https://bugzilla.kernel.org/show_bug.cgi?id=100171] The wake up sequence lasts something like 25 s.