On Wayland, x11grab won't work because there is no direct access to the display like in Xorg. Instead, you'll need to use PipeWire and wf-recorder or obs-v4l2sink.

Solution Using PipeWire (wf-recorder and v4l2loopback)

1. Install required dependencies (if not already installed):

sudo apt install wf-recorder v4l2loopback-utils v4l2loopback-dkms

2. Load the v4l2loopback module (if not already loaded):

sudo modprobe v4l2loopback devices=1 video_nr=10 card_label="Virtual Camera" exclusive_caps=1

This creates /dev/video10, which you will use as your virtual camera.

3. Capture the second monitor and send it to /dev/video10:

wf-recorder -o HDMI-A-1 -f - | ffmpeg -i - -f v4l2 -vcodec rawvideo -pix_fmt yuv420p /dev/video10

Replace HDMI-A-1 with your monitor's correct name. Find your available outputs with:

swaymsg -t get_outputs

I tried swaymsg and got this

$ swaymsg -t get_outputs
00:00:00.040 [swaymsg/main.c:494] Unable to retrieve socket path

I tried to find another way to get the same information.  I ended up trying ls /sys/class/drm and found card1-HDMI_A_1 among the contents of that folder.  

Next I tried the wf-recorder command you said to try.  I got the following and Zoom didn't detect a camera.  Do I have something missing?

$ wf-recorder -o HDMI-A-1 -f - | ffmpeg -i - -f v4l2 -vcodec rawvideo -pix_fmt yuv420p /dev/video10
ffmpeg version 6.1.1-3ubuntu5 Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 13 (Ubuntu 13.2.0-23ubuntu3)
  configuration: --prefix=/usr --extra-version=3ubuntu5 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --disable-omx --enable-gnutls --enable-libaom --enable-libass --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libdav1d --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libglslang --enable-libgme --enable-libgsm --enable-libharfbuzz --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzimg --enable-openal --enable-opencl --enable-opengl --disable-sndio --enable-libvpl --disable-libmfx --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-ladspa --enable-libbluray --enable-libjack --enable-libpulse --enable-librabbitmq --enable-librist --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libx264 --enable-libzmq --enable-libzvbi --enable-lv2 --enable-sdl2 --enable-libplacebo --enable-librav1e --enable-pocketsphinx --enable-librsvg --enable-libjxl --enable-shared
  libavutil      58. 29.100 / 58. 29.100
  libavcodec     60. 31.102 / 60. 31.102
  libavformat    60. 16.100 / 60. 16.100
  libavdevice    60.  3.100 / 60.  3.100
  libavfilter     9. 12.100 /  9. 12.100
  libswscale      7.  5.100 /  7.  5.100
  libswresample   4. 12.100 /  4. 12.100
  libpostproc    57.  3.100 / 57.  3.100
compositor doesn't support wlr-screencopy-unstable-v1
[in#0 @ 0x639aee7d2cc0] Error opening input: Invalid data found when processing input
Error opening input file -.
Error opening input files: Invalid data found when processing input