The solution I found is to use either “Load Stream Level” or “Load Level Instance”, and input in the location pin the coordinates of the location at which the ARKIT hit test (which is done with a Line Trace Tracked Objects - plane extents and plane boundary polygon) was successful.
Everything is automagically networked and the player is spawned with normal spawning rules from the gamemode, replications works fine .
Did not really looked into how to materialize it with respect of the progressive growth of the tracked surface.
Hope this helps.
PS. For more info I suggest to look into ARSample for 4.16. It was my inspiration, and once updated a few old functions by trial and error, it ended up being a very clever way of managing tracking/running states.