So in the next version 3.7 you should be able to treat it as a module and thus define the position.
If you want, you can try it already by installing polybar-git from the AUR.
The new feature was already merged into the master branch.
Please note that polybar-git is compiling from the very latest sources. It might not always be stable…
Also that tray feature is still considered to be experimental at this stage.