ChatGPT suggested temporarily disabling the Steam overlay by moving D3D11Overlay_x64.ext.dll out of the game's directory.
Now, Steam Remote Play Together shows the game on the guest, but I still can't play because:
"input is temporarily disabled whilst the host is busy" - but the host isn't busy
So I made a build without the "export for Steam" option, but see the same thing.
ChatGPT seems to suggest that the overlay is required to pass input from the guest back to the host, so I'm currently stumped.