OK I reviewed your previous post with the picture.
The Pi connects to the internet using a serial port to another device. True? In that case, what do the following commands show?
If a Pi running Weaved has a network connection, then it should be possible to contact it remotely. Is there a reason that you don't simply put the Pi's ethernet on the factory LAN?
When you say you can "see the Pi", what connection are you using exactly?
If you are using Weaved to SSH to your Pi, then you should be able to access any device connected to the Pi, whether on GPIO, USB, etc. If you used VNC to connect to the Pi, again you should be able to access anything connected to that Pi.
The point I was making about VPN perhaps doesn't matter. When you use VPN, your local computer and the remote network are joined; in other words your local PC "looks like" it's actually on that LAN's subnet. That does not happen with Weaved. Your local PC and the remote LAN are isolated. You can still accomplish things on the remote LAN, but it is through the connection to the target device which is really on the remote LAN.