Prominent Wayland developer Simon Ser sent out a merge request today for introducing the new ext-transient-output protocol. With ext-transient-output, it's intent is for helping remoting software like VNC servers in a Wayland world.
The ext-transient-output protocol is akin to ext-transient-seat but for Wayland outputs (wl_output). The ext-transient-seat protocol allows privileged clients to create independent seats dynamically. These transient seats paired with virtual keyboard/pointer input protocols are useful for remote desktop / VNC software while ext-transient-output will take care of the dynamic output support for privileged clients. The virtual outputs can then be captured via ext-image-copy-capture and displayed by a remote client.
This merge request posted today lays out the proposed protocol. The protocol proposal is summarized as:
"The transient output protocol can be used by privileged clients to create independent virtual outputs that will be removed from the compositor when the client destroys the object.This protocol is intended for use with output capture protocols such as "ext_output_image_capture_source_manager_v1" for remoting use-cases.
The "wl_output" global created by this protocol is not tied to a particular hardware device."
So far there is an experimental implementation of ext-transient-output for the wlroots compositor library via this draft merge request.