I have been interested into the same some weeks ago. I send the question to the wayland IRC channel.
You can have a look at:
https://people.freedesktop.org/~cbrill/dri-log/index.php?channel=wayland&highlight_names=&date=2017-12-19
https://people.freedesktop.org/~cbrill/dri-log/index.php?channel=wayland&highlight_names=&date=2017-12-20
Here is the relevant part:
22:31 maggu2810: Is there any change to change the display / output position in weston? I am using three outputs but don't know how to configure which one if left / right of the other one.
22:33 maggu2810: ... chance to change ...
22:35 maggu2810: the output sections in weston.ini currently contains the name, the mode and the scale. I didn't find any "position" option or anything that looks like an option to modify the "order" of the displays.
07:07 pq: maggu2810, yes, not implemented yet, there were indeed some WIP patches in 2016
Perhaps you will be able to post a feature request in their mailing list. I don't think they will look for feature requests on SO ;)