does a qmenu work on top the special lockscreen window?

you maybe want to "report the friking thing and put a link" before pushing :)

Turns out it does not.

Which means my options are:

• come up with an in-window pop-up (IIRC there used to be one back in KDE4 days?)

• use click to change (so it behaves differently from SDDM.. (I could change SDDM too but the session chooser still needs to be a list really)
  • something else?

maybe you are referring to PlasmaComponents.CommonDialog? that should even work in plasma5, tough isn't really used much around.
that thing basically prefers an inline dialog without new windows, but creates a plasmaCore.Dialog if the "fake" dialog doesn't fit into the parent window. it *may* work well enough there.

otherwise, the safest thing here is to do a little fake popup, would just be a rectangle with the list of layouts that gets shown/hidden, that i guess both lockscreen and sddm could share without significant drawbacks