XForwarding in particular is actually not really much overhead. In fact remote location of machines is not uncommon in professional studios for a noise standpoint, usually accomplished with cables and extenders though.
But that being said I have looked at what you are asking before. There are a couple of components, one is the audio transport, getting audio from your ‘recording/control’ machine to the server, unless you want to record on the server? There are a few ways to accomplish this, if not on Linux Dante is a fantastic way to do this, Dante Via in particular. If on Linux NetJack can accomplish this, but it may require a bit more work.
Then you have the control itself. Here is where I will come back around to the original point. Look at what would need to be controlled for a full setup, not only the Ardour UI itself but also the UI of every plugin as well which is code outside of Ardour’s control. Ways around this mean hosting the plugins on the control computer which is probably not what you want, and much more work that grows as your session grows. So really you want every aspect of the UI on the control computer… which is exactly what XForwarding in particular does pretty effectively (Little machine overhead), and VNC does in a different way(More machine overhead as the video stream is compressed and decompressed by the respective computers). For Ardour to implement this effectively means implementing the same technology for the plugin UIs as has already been done and probably done more effectively than Ardour’s team could do any time soon. Seeing as probably 90% of Ardour’s code is in the UI, that is a vast undertaking honestly.
To be honest VNC solutions are utilized in professional situations in the live sound world in particular (Where I live) which can include recording in theaters and venues for this fairly commonly as well as KVM extenders and cabling as mentioned above.