To my understanding there's no reason why a Wayland plugin can't hide the app in the local machine, forward it's windows to a remote machine, and feed the app input from the remote machine. Am I wrong?
What a lot of people don't understand about about Wayland is that it's not intended as a complete replacement for everything Xorg does, if it were then it would have no advantage over Xorg. We've become so accustom to the display server protocal, X11, supporting network transparency, font rendering (hardly ever used anymore), drawing of polygons and archs (also hardly ever used anymore)... that we simply expect that that's what a display server does.
Wayland is a display server, and only a display server. It composites buffers to the screen using a very basic protocol, and leaves all other features to a different level, they are out of the scope of Wayland.
One way to think of it is to ask a similar question about the sound architecture in Linux rather than the graphics architecture.
Does ALSA support network transparency? The answer is no, that's outside the scope of ALSA. But you can still use network transparency in linux, that's why we have pulseaudio.
Your question is slightly different, but I think is still based on the idea that this is something Wayland itself should support.
Can ALSA have network transparency implemented as a plugin? I would answer no, because I wouldn't consider pulseaudio to be a "plugin" for ALSA, but rather something that runs on top of ALSA.
So "Can Wayland have network transparency implemented as a plugin"? No, but that doesn't in any way mean that you won't be able to use network transparency with Wayland.
Xorg can run on top of Wayland, and just as efficiently as if Xorg itself were the display server. So you can still use "ssh -X" and get a local window for a remote app, running right alongside all your other local windows that may be native Wayland clients.
So Wayland will in no way prevent you from using X11 applications remotely, exactly as you do today. And in addition it allows efficient implementations of other protocols for network transparency, like RDP, VNC, and even HTML5: http://blogs.gnome.org/alexl/2010/11/23/gtk3-vs-html5/
I hope that short novel answers your question :)
Yes, a Wayland compositor could do this. I don't expect there to be many applications interfacing directly with Wayland rather than through toolkits like Qt/GTK though.