Today I implemented fetching keyboard layout options from org.freedesktop.locale1, used by things like distro installers to configure the layout:
https://github.com/YaLTeR/niri/pull/2052
I guess this is the closest thing to global cross-desktop keyboard layout configuration, so it should also help environments like LXQt integrating niri.
Also recently merged: niri now sets a logind session locked hint, useful for e.g. idle tools that put the PC to sleep, but only once the lockscreen is fully up.