Wayland non-standard display resolutions in KVM guests

How do a set a ‘non-standard’ resolution in Plasma 6/Wayland that I can use in a KVM guest?

In X, from my old notes, I get a mode line with ‘cvt’, for example:

cvt 1784 1022

and create a conf file under xorg.conf.d, which might be /etc/X11/xorg.conf.d/ or /usr/share/X11/xorg.conf.d, containing the info:

Section "Monitor"
    Identifier   "Virtual-0"
    Modeline "1784x1022_60.00"  151.00  1784 1896 2080 2376  1022 1025 1035 1060 -hsync +vsync
    Option "PreferredMode" "1784x1022_60.00"
EndSection
Section "Monitor"
    Identifier   "Virtual-1"
    Modeline "1784x1022_60.00"  151.00  1784 1896 2080 2376  1022 1025 1035 1060 -hsync +vsync
    Option "PreferredMode" "1784x1022_60.00"
EndSection

I can then select this resolution in System Settings > Display and Monitor

Anyone know the equivalent in Wayland?

(As an aside… QXL video seems to offer more resolution choices than Virtio but behaviour with Virtio is more predictable)